Add Int8DynActInt8WeightLinear module (#5605)

Summary:
Pull Request resolved: pytorch/executorch#5605

Adding Int8DynActInt8WeightLinear for Per Channel DQ Linear

Reviewed By: mergennachin

Differential Revision: D63339550

fbshipit-source-id: 032a699a7f7fcc03177215fc40381038b4354c7a

Author: mcr229

examples/models/llama2/source_transformation/quantize.py

@@ -379,6 +379,99 @@ def forward(self, input: torch.Tensor) -> torch.Tensor:
         # return F.linear(input, self.weight.to(dtype=input.dtype)) * se...
 
 
+def linear_forward_8da8w(
+    x,
+    weight_int8,
+    scales,
+    zeros,
+    out_features,
+    precision,
+):
+    from torchao.quantization.utils import per_token_dynamic_quant
+
+    x = per_token_dynamic_quant(x)
+    n_bit = 8
+    quant_min = -(2 ** (n_bit - 1))
+    quant_max = 2 ** (n_bit - 1) - 1
+    w_dq = torch.ops.quantized_decomposed.dequantize_per_channel(
+        weight_int8,
+        scales,
+        zeros,
+        0,
+        quant_min,
+        quant_max,
+        torch.int8,
+        out_dtype=precision,
+    )
+    c = torch.nn.functional.linear(x, w_dq)
+
+    return c
+
+
+class Int8DynActInt8WeightLinear(torch.nn.Module):
+    __constants__ = ["in_features", "out_features"]
+
+    in_features: int
+    out_features: int
+    weight: torch.Tensor
+
+    """
+    This module implements a dynamic quantized linear layer with int8 weight.
+    Weights are per channel quantized. Parameters of importance
+    precision: precision of input and output. e.g. torch.float32 means input
+    activation is float32 and output is float32.
+    """
+
+    def __init__(
+        self,
+        in_features: int,
+        out_features: int,
+        bias=True,
+        device=None,
+        dtype=None,
+        precision: torch.dtype = torch.float32,
+    ) -> None:
+        super().__init__()
+        self.in_features = in_features
+        self.out_features = out_features
+        assert not bias, "require bias=False"
+        self.precision = precision
+
+        if dtype is not None:
+            raise ValueError("Please specify 'precision' instead of 'dtype'")
+
+        # currently storing unpacked int8 weights
+        self.register_buffer(
+            "weight",
+            torch.empty((out_features, in_features), dtype=torch.int8),
+        )
+        self.register_buffer(
+            "scales",
+            torch.empty(
+                (out_features),
+                dtype=torch.float32,
+            ),
+        )
+        self.register_buffer(
+            "zeros",
+            torch.empty(
+                (out_features),
+                dtype=torch.float32,
+            ),
+        )
+
+    def forward(self, input: torch.Tensor) -> torch.Tensor:
+        input = input.to(self.precision)
+        return linear_forward_8da8w(
+            input,
+            self.weight,
+            self.scales,
+            self.zeros,
+            self.out_features,
+            self.precision,
+        )
+
+
 #########################################################################
 #####                   embedding table quantization               ######