Enable headless models for pooling in the Transformers backend (#21767)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

tests/models/registry.py

@@ -525,6 +525,7 @@ _SPECULATIVE_DECODING_EXAMPLE_MODELS = {
 }
 
 _TRANSFORMERS_BACKEND_MODELS = {
+    "TransformersModel": _HfExamplesInfo("Qwen/Qwen3-Embedding-0.6B"),
     "TransformersForCausalLM": _HfExamplesInfo("hmellor/Ilama-3.2-1B", trust_remote_code=True),  # noqa: E501
     "TransformersForMultimodalLM": _HfExamplesInfo("OpenGVLab/InternVL3-1B-hf"),
 }

tests/models/test_transformers.py

@@ -34,8 +34,7 @@ def check_implementation(
 
     with runner_test(model, **kwargs_test, **kwargs) as model_test:
         model_config = model_test.llm.llm_engine.model_config
-        assert model_config.architecture == (
+        assert model_config.using_transformers_backend()
-            model_config._get_transformers_backend_cls())
 
         outputs_test = model_test.generate_greedy_logprobs(*args)
 
@@ -135,8 +134,7 @@ def test_quantization(
             enforce_eager=True,
             **quantization_kwargs) as vllm_model:  # type: ignore[arg-type]
         model_config = vllm_model.llm.llm_engine.model_config
-        assert model_config.architecture == (
+        assert model_config.using_transformers_backend()
-            model_config._get_transformers_backend_cls())
 
         transformers_outputs = vllm_model.generate_greedy_logprobs(
             example_prompts, max_tokens=max_tokens, num_logprobs=num_logprobs)
@@ -149,6 +147,25 @@ def test_quantization(
     )
 
 
+@pytest.mark.parametrize(
+    "model",
+    [
+        # Layers live in `layers`
+        "Qwen/Qwen3-Embedding-0.6B",
+        # Layers live in `model.layers`
+        "meta-llama/Llama-3.2-1B-Instruct"
+    ],
+)
+def test_embed_loading(vllm_runner, model):
+    with vllm_runner(model,
+                     max_model_len=1024,
+                     enforce_eager=True,
+                     runner="pooling",
+                     model_impl="transformers") as model_test:
+        model_config = model_test.llm.llm_engine.model_config
+        assert model_config.using_transformers_backend()
+
+
 @pytest.mark.parametrize(
     "model",
     ["jason9693/Qwen2.5-1.5B-apeach"],
@@ -169,8 +186,7 @@ def test_classify(
                      dtype=dtype,
                      model_impl="transformers") as vllm_model:
         model_config = vllm_model.llm.llm_engine.model_config
-        assert model_config.architecture == (
+        assert model_config.using_transformers_backend()
-            model_config._get_transformers_backend_cls())
 
         vllm_outputs = vllm_model.classify(example_prompts)
 

vllm/config.py

@@ -812,13 +812,18 @@ class ModelConfig:
     def _get_transformers_backend_cls(self) -> str:
         """Determine which Transformers backend class will be used if
         `model_impl` is set to `transformers` or `auto`."""
+        if getattr(self, "runner_type", self.runner) == "pooling":
+            return "TransformersModel"
         if self.hf_config != self.hf_text_config:
             # If 'hf_text_config' is the same as 'hf_config'. If not, it is
             # probably a composite config, i.e. multimodal
             return "TransformersForMultimodalLM"
-        else:
         return "TransformersForCausalLM"
 
+    def using_transformers_backend(self) -> bool:
+        """Check if the model is using the Transformers backend class."""
+        return self.architecture == self._get_transformers_backend_cls()
+
     @property
     def registry(self):
         return me_models.ModelRegistry

vllm/model_executor/models/registry.py

@@ -270,8 +270,9 @@ _TRANSFORMERS_SUPPORTED_MODELS = {
 }
 
 _TRANSFORMERS_BACKEND_MODELS = {
-    "TransformersForMultimodalLM": ("transformers", "TransformersForMultimodalLM"), # noqa: E501
+    "TransformersModel": ("transformers", "TransformersModel"),
     "TransformersForCausalLM": ("transformers", "TransformersForCausalLM"),
+    "TransformersForMultimodalLM": ("transformers", "TransformersForMultimodalLM"), # noqa: E501
 }
 # yapf: enable
 

vllm/model_executor/models/transformers.py

@@ -651,6 +651,18 @@ class TransformersBase(nn.Module, SupportsQuant, SupportsLoRA, SupportsPP):
         return loader.load_weights(weights, mapper=self.hf_to_vllm_mapper)
 
 
+@support_torch_compile
+class TransformersModel(TransformersBase):
+    hf_to_vllm_mapper = WeightsMapper(
+        orig_to_new_prefix={
+            # Add `model.` prefix for base model checkpoints
+            "": "model.",
+            # Remove `model.` from places it should not be
+            "model.model.": "model.",
+            "model.score": "score",
+        })
+
+
 @support_torch_compile
 class TransformersForCausalLM(TransformersBase):