Support qwen 0.5 b (#13972)

Summary:
Add entry point to run 0.5B model. Run with
```
 buck run mode/dev-nosan //executorch/extension/llm/export:export_llm -- --config executorch/examples/models/llama/config/llama_bf16.yaml +base.model_class="qwen2_5_0_5b" +base.params="./executorch/examples/models/qwen2_5/config/0_5b_config.json"
```

Differential Revision: D81724880

Author: cccclai

examples/models/llama/export_llama_lib.py

@@ -93,7 +93,8 @@
     "llama3_1",
     "llama3_2",
     "static_llama",
-    "qwen2_5",
+    "qwen2_5_0_5b",
+    "qwen2_5_1_5b",
     "qwen3_0_6b",
     "qwen3_1_7b",
     "qwen3_4b",
@@ -102,7 +103,8 @@
 ]
 TORCHTUNE_DEFINED_MODELS = ["llama3_2_vision"]
 HUGGING_FACE_REPO_IDS = {
-    "qwen2_5": "Qwen/Qwen2.5-1.5B",
+    "qwen2_5_0_5b": "Qwen/Qwen2.5-0.5B",
+    "qwen2_5_1_5b": "Qwen/Qwen2.5-1.5B",
     "phi_4_mini": "microsoft/Phi-4-mini-instruct",
     "smollm2": "HuggingFaceTB/SmolLM-135M",
     "qwen3_0_6b": "Qwen/Qwen3-0.6B",
@@ -595,7 +597,7 @@ def export_llama(
     model_name = llm_config.base.model_class.value
     if not llm_config.base.checkpoint and model_name in HUGGING_FACE_REPO_IDS:
         repo_id = HUGGING_FACE_REPO_IDS[model_name]
-        if model_name == "qwen2_5":
+        if model_name.startswith("qwen2_5"):
             from executorch.examples.models.qwen2_5 import convert_weights
         elif model_name.startswith("qwen3"):
             from executorch.examples.models.qwen3 import convert_weights

examples/models/qwen2_5/README.md

@@ -6,9 +6,10 @@ Qwen 2.5 is the latest iteration of the Qwen series of large language models (LL
 Qwen 2.5 uses the same example code as Llama, while the checkpoint, model params, and tokenizer are different. Please see the [Llama README page](../llama/README.md) for details.
 
 All commands for exporting and running Llama on various backends should also be applicable to Qwen 2.5, by swapping the following args:
+
 ```
-base.model_class="qwen2_5"
-base.params="examples/models/qwen2_5/config/1_5b_config.json"
+base.model_class=[qwen2_5_0_5b, qwen2_5_1_5b]
+base.params=[examples/models/qwen2_5/config/0_5b_config.json, examples/models/qwen2_5/config/1_5b_config.json]
 base.checkpoint=<path-to-meta-checkpoint>
 ```
 
@@ -34,7 +35,7 @@ QWEN_CHECKPOINT=path/to/checkpoint.pth
 
 python -m extension.llm.export.export_llm \
   --config examples/models/qwen2_5/config/qwen2_5_xnnpack_q8da4w.yaml \
-  +base.model_class="qwen2_5" \
+  +base.model_class="qwen2_5_1_5b" \
   +base.checkpoint="${QWEN_CHECKPOINT:?}" \
   +base.params="examples/models/qwen2_5/config/1_5b_config.json" \
   +export.output_name="qwen2_5-1_5b.pte" \

extension/llm/export/config/llm_config.py

@@ -37,7 +37,8 @@ class ModelType(str, Enum):
     llama3_2 = "llama3_2"
     llama3_2_vision = "llama3_2_vision"
     static_llama = "static_llama"
-    qwen2_5 = "qwen2_5"
+    qwen2_5_0_5b = "qwen2_5_0_5b"
+    qwen2_5_1_5b = "qwen2_5_1_5b"
     qwen3_0_6b = "qwen3_0_6b"
     qwen3_1_7b = "qwen3_1_7b"
     qwen3_4b = "qwen3_4b"