New multi-step QAT API (#2629)

* [bc-breaking] Generalize FakeQuantizeConfig beyond intx

 **Summary:** The existing `FakeQuantizeConfig` performs only
intx quantization, but we plan to extend QAT to other dtypes
such as fp8 and nvfp4 in the near future. This is the necessary
refactor before that. Specifically:

```
# New abstract class
FakeQuantizeConfigBase
# Rename
FakeQuantizeConfig -> IntxFakeQuantizeConfig
```

In the future, we will have other types of `FakeQuantizeConfigBase`
for float dtypes that users can pass in instead of the existing
Intx one.

**BC-breaking notes:** For BC, we keep around the old names to
reference the new ones. However, this commit is still BC-breaking
in the sense that a few APIs now accept the abstract
`FakeQuantizeConfigBase` instead. For the most part, this abstract
class will be hidden from the user.

Before:
```
activation_config = FakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = FakeQuantizeConfig(torch.int4, group_size=32)
```

After:
```
activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
```

**Test Plan:**
python test/quantization/test_qat.py

[ghstack-poisoned]

* New multi-step QAT API

**Summary:** This commit adds a new multi-step QAT API with the
main goal of simplifying the existing UX. The new API uses the
same `QATConfig` for both the prepare and convert steps, and
automatically infers the fake quantization configs based on
a PTQ base config provided by the user:

```
from torchao.quantization import (
    quantize_,
    Int8DynamicActivationInt4WeightConfig
)
from torchao.quantization.qat import QATConfig

\# prepare
base_config = Int8DynamicActivationInt4WeightConfig(group_size=32)
qat_config = QATConfig(base_config, step="prepare")
quantize_(m, qat_config)

\# train (not shown)

\# convert
quantize_(m, QATConfig(base_config, step="convert"))
```

The main improvements include:
- A single config for both prepare and convert steps
- A single quantize_ for convert (instead of 2)
- No chance for incompatible prepare vs convert configs
- Much less boilerplate code for most common use case
- Simpler config names

For less common use cases such as experimentation, users can
still specify arbitrary fake quantization configs for
activations and/or weights as before. This is still important
since there may not always be a corresponding PTQ base config.
For example:

```
from torchao.quantization import quantize_
from torchao.quantization.qat import IntxFakeQuantizeConfig, QATConfig

activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = QATConfig(
    activation_config=activation_config,
    weight_config=weight_config,
    step="prepare",
)
quantize_(model, qat_config)

\# train and convert same as above (not shown)
```

**BC-breaking notes:** This change by itself is technically not
BC-breaking since we keep around the old path, but will become
so when we deprecate and remove the old path in the future.

Before:
```
\# prepare
activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = IntXQuantizationAwareTrainingConfig(activation_config, weight_config),
quantize_(model, qat_config)

\# train (not shown)

\# convert
quantize_(model, FromIntXQuantizationAwareTrainingConfig())
quantize_(model, Int8DynamicActivationInt4WeightConfig(group_size=32))
```

After: (see above)

**Test Plan:**
```
python test/quantization/test_qat.py
```

[ghstack-poisoned]

* Update on "New multi-step QAT API"


**Summary:** This commit adds a new multi-step QAT API with the
main goal of simplifying the existing UX. The new API uses the
same `QATConfig` for both the prepare and convert steps, and
automatically infers the fake quantization configs based on
a PTQ base config provided by the user:

```
from torchao.quantization import (
    quantize_,
    Int8DynamicActivationInt4WeightConfig
)
from torchao.quantization.qat import QATConfig

# prepare
base_config = Int8DynamicActivationInt4WeightConfig(group_size=32)
quantize_(m, QATConfig(base_config, step="prepare"))

# train (not shown)

# convert
quantize_(m, QATConfig(base_config, step="convert"))
```

The main improvements include:
- A single config for both prepare and convert steps
- A single quantize_ for convert (instead of 2)
- No chance for incompatible prepare vs convert configs
- Much less boilerplate code for most common use case
- Simpler config names

For less common use cases such as experimentation, users can
still specify arbitrary fake quantization configs for
activations and/or weights as before. This is still important
since there may not always be a corresponding PTQ base config.
For example:

```
from torchao.quantization import quantize_
from torchao.quantization.qat import IntxFakeQuantizeConfig, QATConfig

# prepare
activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = QATConfig(
    activation_config=activation_config,
    weight_config=weight_config,
    step="prepare",
)
quantize_(model, qat_config)

# train and convert same as above (not shown)
```

**BC-breaking notes:** This change by itself is technically not
BC-breaking since we keep around the old path, but will become
so when we deprecate and remove the old path in the future.

Before:
```
# prepare
activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = IntXQuantizationAwareTrainingConfig(activation_config, weight_config),
quantize_(model, qat_config)

# train (not shown)

# convert
quantize_(model, FromIntXQuantizationAwareTrainingConfig())
quantize_(model, Int8DynamicActivationInt4WeightConfig(group_size=32))
```

After: (see above)

**Test Plan:**
```
python test/quantization/test_qat.py
```

[ghstack-poisoned]

* Update on "New multi-step QAT API"


**Summary:** This commit adds a new multi-step QAT API with the
main goal of simplifying the existing UX. The new API uses the
same `QATConfig` for both the prepare and convert steps, and
automatically infers the fake quantization configs based on
a PTQ base config provided by the user:

```
from torchao.quantization import (
    quantize_,
    Int8DynamicActivationInt4WeightConfig
)
from torchao.quantization.qat import QATConfig

# prepare
base_config = Int8DynamicActivationInt4WeightConfig(group_size=32)
quantize_(m, QATConfig(base_config, step="prepare"))

# train (not shown)

# convert
quantize_(m, QATConfig(base_config, step="convert"))
```

The main improvements include:
- A single config for both prepare and convert steps
- A single quantize_ for convert (instead of 2)
- No chance for incompatible prepare vs convert configs
- Much less boilerplate code for most common use case
- Simpler config names

For less common use cases such as experimentation, users can
still specify arbitrary fake quantization configs for
activations and/or weights as before. This is still important
since there may not always be a corresponding PTQ base config.
For example:

```
from torchao.quantization import quantize_
from torchao.quantization.qat import IntxFakeQuantizeConfig, QATConfig

# prepare
activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = QATConfig(
    activation_config=activation_config,
    weight_config=weight_config,
    step="prepare",
)
quantize_(model, qat_config)

# train and convert same as above (not shown)
```

**BC-breaking notes:** This change by itself is technically not
BC-breaking since we keep around the old path, but will become
so when we deprecate and remove the old path in the future.

Before:
```
# prepare
activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = IntXQuantizationAwareTrainingConfig(activation_config, weight_config),
quantize_(model, qat_config)

# train (not shown)

# convert
quantize_(model, FromIntXQuantizationAwareTrainingConfig())
quantize_(model, Int8DynamicActivationInt4WeightConfig(group_size=32))
```

After: (see above)

**Test Plan:**
```
python test/quantization/test_qat.py
```

[ghstack-poisoned]

* Update base for Update on "New multi-step QAT API"


**Summary:** This commit adds a new multi-step QAT API with the
main goal of simplifying the existing UX. The new API uses the
same `QATConfig` for both the prepare and convert steps, and
automatically infers the fake quantization configs based on
a PTQ base config provided by the user:

```
from torchao.quantization import (
    quantize_,
    Int8DynamicActivationInt4WeightConfig
)
from torchao.quantization.qat import QATConfig

\# prepare
base_config = Int8DynamicActivationInt4WeightConfig(group_size=32)
qat_config = QATConfig(base_config, step="prepare")
quantize_(m, qat_config)

\# train (not shown)

\# convert
quantize_(m, QATConfig(base_config, step="convert"))
```

The main improvements include:
- A single config for both prepare and convert steps
- A single quantize_ for convert (instead of 2)
- No chance for incompatible prepare vs convert configs
- Much less boilerplate code for most common use case
- Simpler config names

For less common use cases such as experimentation, users can
still specify arbitrary fake quantization configs for
activations and/or weights as before. This is still important
since there may not always be a corresponding PTQ base config.
For example:

```
from torchao.quantization import quantize_
from torchao.quantization.qat import IntxFakeQuantizeConfig, QATConfig

activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = QATConfig(
    activation_config=activation_config,
    weight_config=weight_config,
    step="prepare",
)
quantize_(model, qat_config)

\# train and convert same as above (not shown)
```

**BC-breaking notes:** This change by itself is technically not
BC-breaking since we keep around the old path, but will become
so when we deprecate and remove the old path in the future.

Before:
```
\# prepare
activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = IntXQuantizationAwareTrainingConfig(activation_config, weight_config),
quantize_(model, qat_config)

\# train (not shown)

\# convert
quantize_(model, FromIntXQuantizationAwareTrainingConfig())
quantize_(model, Int8DynamicActivationInt4WeightConfig(group_size=32))
```

After: (see above)

**Test Plan:**
```
python test/quantization/test_qat.py
```

[ghstack-poisoned]

* Update base for Update on "New multi-step QAT API"


**Summary:** This commit adds a new multi-step QAT API with the
main goal of simplifying the existing UX. The new API uses the
same `QATConfig` for both the prepare and convert steps, and
automatically infers the fake quantization configs based on
a PTQ base config provided by the user:

```
from torchao.quantization import (
    quantize_,
    Int8DynamicActivationInt4WeightConfig
)
from torchao.quantization.qat import QATConfig

\# prepare
base_config = Int8DynamicActivationInt4WeightConfig(group_size=32)
qat_config = QATConfig(base_config, step="prepare")
quantize_(m, qat_config)

\# train (not shown)

\# convert
quantize_(m, QATConfig(base_config, step="convert"))
```

The main improvements include:
- A single config for both prepare and convert steps
- A single quantize_ for convert (instead of 2)
- No chance for incompatible prepare vs convert configs
- Much less boilerplate code for most common use case
- Simpler config names

For less common use cases such as experimentation, users can
still specify arbitrary fake quantization configs for
activations and/or weights as before. This is still important
since there may not always be a corresponding PTQ base config.
For example:

```
from torchao.quantization import quantize_
from torchao.quantization.qat import IntxFakeQuantizeConfig, QATConfig

activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = QATConfig(
    activation_config=activation_config,
    weight_config=weight_config,
    step="prepare",
)
quantize_(model, qat_config)

\# train and convert same as above (not shown)
```

**BC-breaking notes:** This change by itself is technically not
BC-breaking since we keep around the old path, but will become
so when we deprecate and remove the old path in the future.

Before:
```
\# prepare
activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = IntXQuantizationAwareTrainingConfig(activation_config, weight_config),
quantize_(model, qat_config)

\# train (not shown)

\# convert
quantize_(model, FromIntXQuantizationAwareTrainingConfig())
quantize_(model, Int8DynamicActivationInt4WeightConfig(group_size=32))
```

After: (see above)

**Test Plan:**
```
python test/quantization/test_qat.py
```

[ghstack-poisoned]

* Update base for Update on "New multi-step QAT API"


**Summary:** This commit adds a new multi-step QAT API with the
main goal of simplifying the existing UX. The new API uses the
same `QATConfig` for both the prepare and convert steps, and
automatically infers the fake quantization configs based on
a PTQ base config provided by the user:

```Py
from torchao.quantization import (
    quantize_,
    Int8DynamicActivationInt4WeightConfig
)
from torchao.quantization.qat import QATConfig

\# prepare
base_config = Int8DynamicActivationInt4WeightConfig(group_size=32)
qat_config = QATConfig(base_config, step="prepare")
quantize_(m, qat_config)

\# train (not shown)

\# convert
quantize_(m, QATConfig(base_config, step="convert"))
```

The main improvements include:
- A single config for both prepare and convert steps
- A single quantize_ for convert (instead of 2)
- No chance for incompatible prepare vs convert configs
- Much less boilerplate code for most common use case
- Simpler config names

For less common use cases such as experimentation, users can
still specify arbitrary fake quantization configs for
activations and/or weights as before. This is still important
since there may not always be a corresponding PTQ base config.
For example:

```Py
from torchao.quantization import quantize_
from torchao.quantization.qat import IntxFakeQuantizeConfig, QATConfig

activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = QATConfig(
    activation_config=activation_config,
    weight_config=weight_config,
    step="prepare",
)
quantize_(model, qat_config)

\# train and convert same as above (not shown)
```

**BC-breaking notes:** This change by itself is technically not
BC-breaking since we keep around the old path, but will become
so when we deprecate and remove the old path in the future.

Before:
```Py
\# prepare
activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = IntXQuantizationAwareTrainingConfig(activation_config, weight_config),
quantize_(model, qat_config)

\# train (not shown)

\# convert
quantize_(model, FromIntXQuantizationAwareTrainingConfig())
quantize_(model, Int8DynamicActivationInt4WeightConfig(group_size=32))
```

After: (see above)

**Test Plan:**
```
python test/quantization/test_qat.py
```

[ghstack-poisoned]

* Update base for Update on "New multi-step QAT API"


**Summary:** This commit adds a new multi-step QAT API with the
main goal of simplifying the existing UX. The new API uses the
same `QATConfig` for both the prepare and convert steps, and
automatically infers the fake quantization configs based on
a PTQ base config provided by the user:

```Py
from torchao.quantization import (
    quantize_,
    Int8DynamicActivationInt4WeightConfig
)
from torchao.quantization.qat import QATConfig

\# prepare
base_config = Int8DynamicActivationInt4WeightConfig(group_size=32)
qat_config = QATConfig(base_config, step="prepare")
quantize_(m, qat_config)

\# train (not shown)

\# convert
quantize_(m, QATConfig(base_config, step="convert"))
```

The main improvements include:
- A single config for both prepare and convert steps
- A single quantize_ for convert (instead of 2)
- No chance for incompatible prepare vs convert configs
- Much less boilerplate code for most common use case
- Simpler config names

For less common use cases such as experimentation, users can
still specify arbitrary fake quantization configs for
activations and/or weights as before. This is still important
since there may not always be a corresponding PTQ base config.
For example:

```Py
from torchao.quantization import quantize_
from torchao.quantization.qat import IntxFakeQuantizeConfig, QATConfig

activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = QATConfig(
    activation_config=activation_config,
    weight_config=weight_config,
    step="prepare",
)
quantize_(model, qat_config)

\# train and convert same as above (not shown)
```

**BC-breaking notes:** This change by itself is technically not
BC-breaking since we keep around the old path, but will become
so when we deprecate and remove the old path in the future.

Before:
```Py
\# prepare
activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
qat_config = IntXQuantizationAwareTrainingConfig(activation_config, weight_config),
quantize_(model, qat_config)

\# train (not shown)

\# convert
quantize_(model, FromIntXQuantizationAwareTrainingConfig())
quantize_(model, Int8DynamicActivationInt4WeightConfig(group_size=32))
```

After: (see above)

**Test Plan:**
```
python test/quantization/test_qat.py
```

[ghstack-poisoned]

Author: andrewor14

README.md

@@ -179,12 +179,17 @@ With this quantization flow, we achieve **67% VRAM reduction and 12-20% speedup*
 Post-training quantization can result in a fast and compact model, but may also lead to accuracy degradation. We recommend exploring Quantization-Aware Training (QAT) to overcome this limitation, especially for lower bit-width dtypes such as int4. In collaboration with [TorchTune](https://github.com/pytorch/torchtune/blob/main/recipes/quantization.md#quantization-aware-training-qat), we've developed a QAT recipe that demonstrates significant accuracy improvements over traditional PTQ, recovering **96% of the accuracy degradation on hellaswag and 68% of the perplexity degradation on wikitext** for Llama3 compared to post-training quantization (PTQ). For more details, please refer to the [QAT README](torchao/quantization/qat/README.md) and the [original blog](https://pytorch.org/blog/quantization-aware-training/):
 
 ```python
-from torchao.quantization import quantize_
-from torchao.quantization.qat import IntxFakeQuantizeConfig, IntXQuantizationAwareTrainingConfig
-activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
-weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
-qat_config = IntXQuantizationAwareTrainingConfig(activation_config, weight_config),
-quantize_(my_model, qat_config)
+from torchao.quantization import quantize_, Int8DynamicActivationInt4WeightConfig
+from torchao.quantization.qat import QATConfig
+
+# prepare
+base_config = Int8DynamicActivationInt4WeightConfig(group_size=32)
+quantize_(my_model, QATConfig(base_config, step="prepare"))
+
+# train model (not shown)
+
+# convert
+quantize_(my_model, QATConfig(base_config, step="convert"))
 ```
 
 Users can also combine LoRA + QAT to speed up training by [1.89x](https://dev-discuss.pytorch.org/t/speeding-up-qat-by-1-89x-with-lora/2700) compared to vanilla QAT using this [fine-tuning recipe](https://github.com/pytorch/torchtune/blob/main/recipes/qat_lora_finetune_distributed.py).

docs/source/api_ref_qat.rst

@@ -6,7 +6,7 @@ torchao.quantization.qat
 
 .. currentmodule:: torchao.quantization.qat
 
-QAT Configs for quantize_
+Main Config for quantize_
 ---------------------------------------
 For a full example of how to use QAT with our main `quantize_` API,
 please refer to the `QAT README <https://github.com/pytorch/ao/blob/main/torchao/quantization/qat/README.md#quantize_-api-recommended>`__.
@@ -15,29 +15,32 @@ please refer to the `QAT README <https://github.com/pytorch/ao/blob/main/torchao
     :toctree: generated/
     :nosignatures:
 
-    IntXQuantizationAwareTrainingConfig
-    FromIntXQuantizationAwareTrainingConfig
+    QATConfig
+    QATStep
 
 Custom QAT APIs
 ---------------
 .. autosummary::
     :toctree: generated/
     :nosignatures:
 
+    FakeQuantizeConfigBase
     IntxFakeQuantizeConfig
     FakeQuantizedLinear
     FakeQuantizedEmbedding
     FakeQuantizer
     linear.enable_linear_fake_quant
     linear.disable_linear_fake_quant
 
-Legacy QAT Quantizers
+Legacy QAT APIs
 ---------------------
 
 .. autosummary::
     :toctree: generated/
     :nosignatures:
 
+    IntXQuantizationAwareTrainingConfig
+    FromIntXQuantizationAwareTrainingConfig
     Int4WeightOnlyQATQuantizer
     linear.Int4WeightOnlyQATLinear
     Int8DynActInt4WeightQATQuantizer

docs/source/finetuning.rst

@@ -205,21 +205,14 @@ because we are not actually casting the fake quantized values.
 
 .. code:: py
 
-  from torchao.quantization import (
-      quantize_,
-  )
-  from torchao.quantization.qat import (
-      FakeQuantizeConfig,
-      IntXQuantizationAwareTrainingConfig,
-  )
+  from torchao.quantization import quantize_, Int8DynamicActivationInt4WeightConfig
+  from torchao.quantization.qat import QATConfig
+
   model = get_model()
 
-  # prepare: insert fake quantization ops
-  # swaps `torch.nn.Linear` with `FakeQuantizedLinear`
-  activation_config = FakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
-  weight_config = FakeQuantizeConfig(torch.int4, group_size=32)
-  qat_config = IntXQuantizationAwareTrainingConfig(activation_config, weight_config)
-  quantize_(model, qat_config)
+  # prepare: swap `torch.nn.Linear` -> `FakeQuantizedLinear`
+  base_config = Int8DynamicActivationInt4WeightConfig(group_size=32)
+  quantize_(model, QATConfig(base_config, step="prepare"))
 
   # fine-tune
   train_loop(model)
@@ -232,18 +225,12 @@ The next step is to actually quantize the model:
 
 .. code:: py
 
-  from torchao.quantization import (
-      Int8DynamicActivationInt4WeightConfig,
-  )
-  from torchao.quantization.qat import (
-      FromIntXQuantizationAwareTrainingConfig,
-  )
+  from torchao.quantization import Int8DynamicActivationInt4WeightConfig
 
-  # convert: transform fake quantization ops into actual quantized ops
-  # swap `FakeQuantizedLinear` back to `torch.nn.Linear` and inserts
-  # quantized activation and weight tensor subclasses
-  quantize_(model, FromIntXQuantizationAwareTrainingConfig())
-  quantize_(model, Int8DynamicActivationInt4WeightConfig(group_size=32))
+  # convert: swap `FakeQuantizedLinear` -> `torch.nn.Linear`, then quantize using `base_config`
+  quantize_(model, QATConfig(base_config, step="convert"))
+
+  # inference or generate
 
 Now our model is ready for serving, and will typically have higher quantized
 accuracy than if we did not apply the prepare step (fake quantization) during

test/quantization/test_qat.py

@@ -34,6 +34,8 @@
     ComposableQATQuantizer,
     FromIntXQuantizationAwareTrainingConfig,
     IntXQuantizationAwareTrainingConfig,
+    QATConfig,
+    QATStep,
     initialize_fake_quantizers,
 )
 from torchao.quantization.qat.embedding import (
@@ -59,7 +61,7 @@
     _get_qmin_qmax,
 )
 from torchao.quantization.quant_api import (
-    int8_dynamic_activation_int4_weight,
+    Int8DynamicActivationInt4WeightConfig,
 )
 from torchao.quantization.quant_primitives import (
     MappingType,
@@ -1261,11 +1263,67 @@ def test_qat_prototype_bc(self):
     @unittest.skipIf(
         not TORCH_VERSION_AT_LEAST_2_4, "skipping when torch version is 2.4 or lower"
     )
-    def test_quantize_api_standalone(self):
+    def test_qat_config_init(self):
+        """
+        Test that the correct errors are thrown if `QATConfig` is not instantiated properly.
+        """
+        base_config = Int8DynamicActivationInt4WeightConfig(group_size=32)
+        fq_config = IntxFakeQuantizeConfig(torch.int8, "per_channel")
+
+        # OK
+        QATConfig(base_config, step="prepare")
+        QATConfig(base_config, step="convert")
+        QATConfig(base_config, step=QATStep.PREPARE)
+        QATConfig(base_config, step=QATStep.CONVERT)
+        QATConfig(activation_config=fq_config, weight_config=fq_config, step="prepare")
+        QATConfig(weight_config=fq_config, step="prepare")
+
+        # OK: good step values
+        self.assertEqual(QATConfig(base_config).step, "prepare")
+        self.assertEqual(QATConfig(base_config, step="Prepare").step, "prepare")
+        self.assertEqual(QATConfig(base_config, step="CONVERT").step, "convert")
+
+        # Bad step
+        with self.assertRaisesRegex(ValueError, "`step` must be one of"):
+            QATConfig(base_config, step="blah")
+
+        # Step was not a keyword arg
+        with self.assertRaisesRegex(
+            TypeError, "4 positional arguments but 5 were given"
+        ):
+            QATConfig(base_config, None, None, "prepare")
+
+        # No configs are provided
+        with self.assertRaisesRegex(
+            ValueError, "One of `base_config` or `weight_config` must be specified"
+        ):
+            QATConfig(step="prepare")
+
+        # Clashing configs are provided
+        with self.assertRaisesRegex(ValueError, "Cannot specify both"):
+            QATConfig(base_config, weight_config=fq_config, step="prepare")
+        with self.assertRaisesRegex(ValueError, "Cannot specify both"):
+            QATConfig(base_config, activation_config=fq_config, step="prepare")
+        with self.assertRaisesRegex(
+            ValueError, "must be specified in the convert step"
+        ):
+            QATConfig(weight_config=fq_config, step="convert")
+
+        # FakeQuantizeConfigBase was specified as base_config
+        with self.assertRaisesRegex(
+            ValueError,
+            "was passed as `base_config`. Did you mean to do the following instead?",
+        ):
+            QATConfig(fq_config, step="prepare")
+
+    @unittest.skipIf(
+        not TORCH_VERSION_AT_LEAST_2_4, "skipping when torch version is 2.4 or lower"
+    )
+    def test_quantize_api_prepare(self):
         """
         Test that the following:
 
-            quantize_(model, IntXQuantizationAwareTrainingConfig(...))
+            quantize_(model, QATConfig(...))
 
         can produce the same results as `ComposableQATQuantizer`.
         """
@@ -1290,20 +1348,15 @@ def test_quantize_api_standalone(self):
         baseline_model = baseline_quantizer.prepare(baseline_model)
 
         # quantize_ API
-        activation_config = IntxFakeQuantizeConfig(
-            torch.int8,
-            "per_token",
-            is_symmetric=False,
-        )
+        act_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
         weight_config = IntxFakeQuantizeConfig(TorchAODType.INT4, group_size=group_size)
-        quantize_(
-            m,
-            IntXQuantizationAwareTrainingConfig(activation_config, weight_config),
+        qat_config1 = QATConfig(
+            activation_config=act_config, weight_config=weight_config
         )
+        qat_config2 = QATConfig(weight_config=weight_config)
+        quantize_(m, qat_config1)
         quantize_(
-            m,
-            IntXQuantizationAwareTrainingConfig(weight_config=weight_config),
-            filter_fn=lambda m, _: isinstance(m, torch.nn.Embedding),
+            m, qat_config2, filter_fn=lambda m, _: isinstance(m, torch.nn.Embedding)
         )
 
         # Compare model values
@@ -1322,37 +1375,29 @@ def test_quantize_api_errors(self):
         Test that we throw exceptions with helpful error messages if `quantize_`
         runs into unexpected configurations.
         """
-        my_config = IntxFakeQuantizeConfig(torch.int8, group_size=32)
+        fq_config = IntxFakeQuantizeConfig(torch.int8, group_size=32)
+        qat_config = QATConfig(activation_config=fq_config, weight_config=fq_config)
         m = M3()
 
         # Embedding currently only supports weight-only quantization
         with self.assertRaisesRegex(
             ValueError, "Activation fake quantization is not supported for embedding"
         ):
-            quantize_(
-                m,
-                IntXQuantizationAwareTrainingConfig(my_config, my_config),
-                lambda m, _: isinstance(m, torch.nn.Embedding),
-            )
+            quantize_(m, qat_config, lambda m, _: isinstance(m, torch.nn.Embedding))
 
         # Only linear and embedding are supported currently
         with self.assertRaisesRegex(ValueError, "does not have QAT support"):
-            quantize_(
-                m,
-                IntXQuantizationAwareTrainingConfig(my_config, my_config),
-                lambda m, _: isinstance(m, torch.nn.ReLU),
-            )
+            quantize_(m, qat_config, lambda m, _: isinstance(m, torch.nn.ReLU))
 
     @unittest.skipIf(
         not TORCH_VERSION_AT_LEAST_2_4, "skipping when torch version is 2.4 or lower"
     )
-    def test_quantize_api_convert_path(self):
+    def test_quantize_api_e2e(self):
         """
         Test that the following:
 
-            quantize_(model, IntXQuantizationAwareTrainingConfig(...))
-            quantize_(model, FromIntXQuantizationAwareTrainingConfig(...))
-            quantize_(model, int8_dynamic_activation_int4_weight())
+            quantize_(model, QATConfig(Int8DynamicActivationInt4WeightConfig(), step="prepare"))
+            quantize_(model, QATConfig(Int8DynamicActivationInt4WeightConfig(), step="convert"))
 
         can produce the same results as `Int8DynActInt4WeightQATQuantizer` prepare + convert.
         """
@@ -1370,16 +1415,8 @@ def test_quantize_api_convert_path(self):
         baseline_model = baseline_quantizer.prepare(baseline_model)
 
         # quantize_ prepare
-        activation_config = IntxFakeQuantizeConfig(
-            torch.int8,
-            "per_token",
-            is_symmetric=False,
-        )
-        weight_config = IntxFakeQuantizeConfig(TorchAODType.INT4, group_size=group_size)
-        quantize_(
-            m,
-            IntXQuantizationAwareTrainingConfig(activation_config, weight_config),
-        )
+        base_config = Int8DynamicActivationInt4WeightConfig(group_size=group_size)
+        quantize_(m, QATConfig(base_config, step="prepare"))
 
         # Compare prepared values
         torch.manual_seed(self.SEED)
@@ -1393,8 +1430,7 @@ def test_quantize_api_convert_path(self):
         baseline_model = baseline_quantizer.convert(baseline_model)
 
         # quantize_ convert
-        quantize_(m, FromIntXQuantizationAwareTrainingConfig())
-        quantize_(m, int8_dynamic_activation_int4_weight(group_size=group_size))
+        quantize_(m, QATConfig(base_config, step="convert"))
 
         # Compare converted values
         torch.manual_seed(self.SEED)
@@ -1447,14 +1483,12 @@ def test_qat_linear_bias(self):
         Test that QAT supports linear bias.
         """
         m = ModelWithLinearBias()
-        activation_config = IntxFakeQuantizeConfig(
-            torch.int8, "per_token", is_symmetric=False
-        )
+        act_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
         weight_config = IntxFakeQuantizeConfig(TorchAODType.INT4, group_size=32)
-        quantize_(
-            m,
-            IntXQuantizationAwareTrainingConfig(activation_config, weight_config),
+        qat_config = QATConfig(
+            activation_config=act_config, weight_config=weight_config
         )
+        quantize_(m, qat_config)
         example_inputs = m.example_inputs()
         m(*example_inputs)
 
@@ -1653,7 +1687,7 @@ def test_qat_range_learning(self):
         )
         m = M()
         example_inputs = m.example_inputs()
-        quantize_(m, IntXQuantizationAwareTrainingConfig(weight_config=config))
+        quantize_(m, QATConfig(weight_config=config))
 
         # Not initialized, should fail
         for t in m._get_all_weight_qparams():
@@ -1756,6 +1790,60 @@ def test_qat_fp8a4w_quantizer(self):
         self.assertNotEqual(torch.count_nonzero(new_weight.grad), 0)
         self.assertFalse(torch.equal(new_weight, prev_weight))
 
+    @unittest.skipIf(
+        not TORCH_VERSION_AT_LEAST_2_4, "skipping when torch version is 2.4 or lower"
+    )
+    def test_legacy_quantize_api_e2e(self):
+        """
+        Test that the following two APIs are numerically equivalent:
+
+        New API:
+            quantize_(model, QATConfig(Int8DynamicActivationInt4WeightConfig(), step="prepare"))
+            quantize_(model, QATConfig(Int8DynamicActivationInt4WeightConfig(), step="convert"))
+
+        Old API:
+            quantize_(model, IntXQuantizationAwareTrainingConfig(...))
+            quantize_(model, FromIntXQuantizationAwareTrainingConfig())
+            quantize_(model, Int8DynamicActivationInt4WeightConfig())
+        """
+        group_size = 16
+        torch.manual_seed(self.SEED)
+        m = M()
+        baseline_model = copy.deepcopy(m)
+
+        # Baseline prepare
+        act_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
+        weight_config = IntxFakeQuantizeConfig(TorchAODType.INT4, group_size=group_size)
+        old_qat_config = IntXQuantizationAwareTrainingConfig(act_config, weight_config)
+        quantize_(baseline_model, old_qat_config)
+
+        # QATConfig prepare
+        base_config = Int8DynamicActivationInt4WeightConfig(group_size=group_size)
+        quantize_(m, QATConfig(base_config, step="prepare"))
+
+        # Compare prepared values
+        torch.manual_seed(self.SEED)
+        x = m.example_inputs()
+        x2 = copy.deepcopy(x)
+        out = m(*x)
+        baseline_out = baseline_model(*x2)
+        torch.testing.assert_close(out, baseline_out, atol=0, rtol=0)
+
+        # Baseline convert
+        quantize_(baseline_model, FromIntXQuantizationAwareTrainingConfig())
+        quantize_(baseline_model, base_config)
+
+        # quantize_ convert
+        quantize_(m, QATConfig(base_config, step="convert"))
+
+        # Compare converted values
+        torch.manual_seed(self.SEED)
+        x = m.example_inputs()
+        x2 = copy.deepcopy(x)
+        out = m(*x)
+        baseline_out = baseline_model(*x2)
+        torch.testing.assert_close(out, baseline_out, atol=0, rtol=0)
+
 
 if __name__ == "__main__":
     unittest.main()