[Model] Support VLMs with transformers backend

docs/models/supported_models.md

@@ -18,7 +18,7 @@ These models are what we list in [supported-text-models][supported-text-models]
 
 ### Transformers
 
-vLLM also supports model implementations that are available in Transformers. This does not currently work for all models, but most decoder language models are supported, and vision language model support is planned!
+vLLM also supports model implementations that are available in Transformers. This does not currently work for all models, but most decoder language models and common vision language models are supported! Vision-language models currently accept only image inputs, and require setting `--disable_mm_preprocessor_cache` when running. Support for video inputs and caching of multi-modal preprocessors will be added in future releases.
 
 To check if the modeling backend is Transformers, you can simply do this:
 
@@ -28,14 +28,17 @@ llm = LLM(model=..., task="generate")  # Name or path of your model
 llm.apply_model(lambda model: print(type(model)))
 ```
 
-If it is `TransformersForCausalLM` then it means it's based on Transformers!
+If it is `TransformersForCausalLM` or `TransformersForMultimodalLM` then it means it's based on Transformers!
 
 !!! tip
     You can force the use of `TransformersForCausalLM` by setting `model_impl="transformers"` for [offline-inference](../serving/offline_inference.md) or `--model-impl transformers` for the [openai-compatible-server](../serving/openai_compatible_server.md).
 
 !!! note
     vLLM may not fully optimise the Transformers implementation so you may see degraded performance if comparing a native model to a Transformers model in vLLM.
 
+!!! note
+    In case of vision language models if you are loading with `dtype="auto"`, vLLM loads the whole model with config's `dtype` if it exists. In contrast the native Transformers will respect the `dtype` attribute of each backbone in the model. That might cause a slight difference in performance.
+
 #### Custom models
 
 If a model is neither supported natively by vLLM or Transformers, it can still be used in vLLM!
@@ -99,7 +102,7 @@ Here is what happens in the background when this model is loaded:
 
 1. The config is loaded.
 2. `MyModel` Python class is loaded from the `auto_map` in config, and we check that the model `is_backend_compatible()`.
-3. `MyModel` is loaded into `TransformersForCausalLM` (see <gh-file:vllm/model_executor/models/transformers.py>) which sets `self.config._attn_implementation = "vllm"` so that vLLM's attention layer is used.
+3. `MyModel` is loaded into `TransformersForCausalLM` or `TransformersForMultimodalLM` (see <gh-file:vllm/model_executor/models/transformers.py>) which sets `self.config._attn_implementation = "vllm"` so that vLLM's attention layer is used.
 
 That's it!
 

tests/models/multimodal/generation/test_common.py

@@ -35,6 +35,8 @@
 REQUIRES_V0_MODELS = [
     # V1 Test: not enough KV cache space in C1.
     "fuyu",
+    # V1 Test: Deadlock issue when processing mm_inputs
+    "llava-onevision-transformers",
 ]
 
 # yapf: disable
@@ -170,6 +172,79 @@
         hf_output_post_proc=model_utils.ultravox_trunc_hf_output,
         marks=[pytest.mark.core_model, pytest.mark.cpu_model],
     ),
+    #### Transformers fallback to test
+    ## To reduce test burden, we only test batching arbitrary image size
+    # Dynamic image length and number of patches
+    "llava-onevision-transformers": VLMTestInfo(
+        models=["llava-hf/llava-onevision-qwen2-0.5b-ov-hf"],
+        test_type=VLMTestType.IMAGE,
+        prompt_formatter=lambda vid_prompt: f"<|im_start|>user\n{vid_prompt}<|im_end|>\n<|im_start|>assistant\n",   # noqa: E501
+        max_model_len=16384,
+        hf_model_kwargs=model_utils.llava_onevision_hf_model_kwargs("llava-hf/llava-onevision-qwen2-0.5b-ov-hf"),   # noqa: E501
+        auto_cls=AutoModelForImageTextToText,
+        vllm_output_post_proc=model_utils.llava_onevision_vllm_to_hf_output,
+        image_size_factors=[(0.25, 0.5, 1.0)],
+        vllm_runner_kwargs={
+            "model_impl": "transformers",
+            "disable_mm_preprocessor_cache": True,
+            "enable_prefix_caching": False,
+        },
+        marks=[pytest.mark.core_model],
+    ),
+    # FIXME(Isotr0py): Enable this test after
+    # https://github.com/huggingface/transformers/pull/39470 released
+    # "idefics3-transformers": VLMTestInfo(
+    #     models=["HuggingFaceTB/SmolVLM-256M-Instruct"],
+    #     test_type=(VLMTestType.IMAGE, VLMTestType.MULTI_IMAGE),
+    #     prompt_formatter=lambda img_prompt:f"<|begin_of_text|>User:{img_prompt}<end_of_utterance>\nAssistant:",  # noqa: E501
+    #     img_idx_to_prompt=lambda idx: "<image>",
+    #     max_model_len=8192,
+    #     max_num_seqs=2,
+    #     auto_cls=AutoModelForImageTextToText,
+    #     hf_output_post_proc=model_utils.idefics3_trunc_hf_output,
+    #     image_size_factors=[(0.25, 0.5, 1.0)],
+    #     vllm_runner_kwargs={
+    #         "model_impl": "transformers",
+    #         "disable_mm_preprocessor_cache": True,
+    #         "enable_prefix_caching": False,
+    #     },
+    #     marks=[pytest.mark.core_model],
+    # ),
+    # Pixel values from processor are not 4D or 5D arrays
+    "qwen2_5_vl-transformers": VLMTestInfo(
+        models=["Qwen/Qwen2.5-VL-3B-Instruct"],
+        test_type=VLMTestType.IMAGE,
+        prompt_formatter=lambda img_prompt: f"<|im_start|>User\n{img_prompt}<|im_end|>\n<|im_start|>assistant\n", # noqa: E501
+        img_idx_to_prompt=lambda idx: "<|vision_start|><|image_pad|><|vision_end|>", # noqa: E501
+        max_model_len=4096,
+        max_num_seqs=2,
+        auto_cls=AutoModelForImageTextToText,
+        vllm_output_post_proc=model_utils.qwen2_vllm_to_hf_output,
+        image_size_factors=[(0.25, 0.2, 0.15)],
+        vllm_runner_kwargs={
+            "model_impl": "transformers",
+            "disable_mm_preprocessor_cache": True,
+            "enable_prefix_caching": False,
+        },
+        marks=[large_gpu_mark(min_gb=32)],
+    ),
+    # Check "auto" with fallback to transformers
+    "internvl-transformers": VLMTestInfo(
+        models=["OpenGVLab/InternVL3-1B-hf"],
+        test_type=(VLMTestType.IMAGE, VLMTestType.MULTI_IMAGE),
+        prompt_formatter=lambda img_prompt: f"<|im_start|>User\n{img_prompt}<|im_end|>\n<|im_start|>Assistant\n", # noqa: E501
+        img_idx_to_prompt=lambda idx: "<IMG_CONTEXT>",
+        max_model_len=4096,
+        use_tokenizer_eos=True,
+        image_size_factors=[(0.25, 0.5, 1.0)],
+        vllm_runner_kwargs={
+            "model_impl": "auto",
+            "disable_mm_preprocessor_cache": True,
+            "enable_prefix_caching": False,
+        },
+        auto_cls=AutoModelForImageTextToText,
+        marks=[pytest.mark.core_model],
+    ),
     #### Extended model tests
     "aria": VLMTestInfo(
         models=["rhymes-ai/Aria"],

tests/models/registry.py

@@ -496,6 +496,7 @@ def check_available_online(
 
 _TRANSFORMERS_MODELS = {
     "TransformersForCausalLM": _HfExamplesInfo("ArthurZ/Ilama-3.2-1B", trust_remote_code=True),  # noqa: E501
+    "TransformersForMultimodalLM": _HfExamplesInfo("OpenGVLab/InternVL3-1B-hf"),
 }
 
 _EXAMPLE_MODELS = {

vllm/config.py

@@ -562,6 +562,10 @@ def __post_init__(self) -> None:
 
             self.task = "embed"
 
+        model_info, arch = self.registry.inspect_model_cls(self.architectures)
+        self._model_info = model_info
+        self._architecture = arch
+
         all_supported_tasks = self._get_supported_tasks(self.task)
         logger.debug("Tasks supported by runner type: %s", all_supported_tasks)
         supported_runner_types = self._get_supported_runner_types(
@@ -587,10 +591,6 @@ def __post_init__(self) -> None:
         else:
             self.truncation_side = "right"
 
-        model_info, arch = self.registry.inspect_model_cls(self.architectures)
-        self._model_info = model_info
-        self._architecture = arch
-
         self.pooler_config = self._init_pooler_config()
 
         self.dtype = _get_and_verify_dtype(
@@ -674,14 +674,33 @@ def validate_model_config_after(self: "ModelConfig") -> "ModelConfig":
                 "max_model_len must be an integer after __post_init__.")
         return self
 
+    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 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"
+
     @property
     def registry(self):
         return me_models.ModelRegistry
 
     @property
     def architectures(self) -> list[str]:
         # architectures in the model config.
-        return getattr(self.hf_config, "architectures", [])
+        architectures = getattr(self.hf_config, "architectures", [])
+        # The registry assumes that it can always inspect the vLLM model class
+        # for a given architecture. This assumption breaks down for the
+        # Transformers backend, which may use a different class depending on
+        # the model type. To work around this, we add the correct Transformers
+        # backend class to the architectures list. We must do this here because
+        # we need access to the `hf_config` to determine the backend class.
+        if self.model_impl != ModelImpl.VLLM.value:
+            architectures.append(self._get_transformers_backend_cls())
+        return architectures
 
     @property
     def architecture(self) -> str:
@@ -827,10 +846,9 @@ def _get_preferred_pooling_task(
             ("EmbeddingModel", "embed"),
             ("RewardModel", "reward"),
         ]
-        _, arch = self.registry.inspect_model_cls(architectures)
 
         for suffix, pref_task in suffix_to_preferred_task:
-            if arch.endswith(suffix):
+            if self.architecture.endswith(suffix):
                 return pref_task
 
         return "embed"
@@ -944,10 +962,10 @@ def _resolve_runner(
             ("EmbeddingModel", "pooling"),
             ("RewardModel", "pooling"),
         ]
-        _, arch = self.registry.inspect_model_cls(self.architectures)
 
         for suffix, pref_runner in suffix_to_preferred_runner:
-            if arch.endswith(suffix) and pref_runner in supported_runner_types:
+            if self.architecture.endswith(
+                    suffix) and pref_runner in supported_runner_types:
                 return pref_runner
 
         if "generate" in supported_runner_types:

vllm/model_executor/model_loader/utils.py

@@ -25,6 +25,7 @@
                                                  as_reward_model,
                                                  as_seq_cls_model)
 from vllm.model_executor.models.interfaces import SupportsQuant
+from vllm.model_executor.models.registry import _TRANSFORMERS_MODELS
 from vllm.utils import is_pin_memory_available
 
 logger = init_logger(__name__)
@@ -169,9 +170,22 @@ def device_loading_context(module: torch.nn.Module,
 
 def resolve_transformers_arch(model_config: ModelConfig,
                               architectures: list[str]):
+    if model_config.model_impl == ModelImpl.VLLM:
+        raise ValueError(
+            "Attempting to resolve architecture from the Transformers library "
+            "but the model implementation is set to vLLM. This should never "
+            "happen.")
+
     for i, arch in enumerate(architectures):
-        if arch == "TransformersForCausalLM":
+        if arch in _TRANSFORMERS_MODELS:
             continue
+
+        if model_config.model_impl == ModelImpl.AUTO:
+            logger.warning(
+                "%s has no vLLM implementation, falling back to Transformers "
+                "implementation. Some features may not be supported and "
+                "performance may not be optimal.", arch)
+
         auto_map: dict[str, str] = getattr(model_config.hf_config, "auto_map",
                                            None) or dict()
         # Make sure that config class is always initialized before model class,
@@ -199,25 +213,13 @@ def resolve_transformers_arch(model_config: ModelConfig,
                     "not present in the model config's 'auto_map' (relevant "
                     "if the model is custom).")
             model_module = auto_modules["AutoModel"]
-        # TODO(Isotr0py): Further clean up these raises.
-        # perhaps handled them in _ModelRegistry._raise_for_unsupported?
-        if model_config.model_impl == ModelImpl.TRANSFORMERS:
-            if not model_module.is_backend_compatible():
-                raise ValueError(
-                    f"The Transformers implementation of {arch} is not "
-                    "compatible with vLLM.")
-            architectures[i] = "TransformersForCausalLM"
-        if model_config.model_impl == ModelImpl.AUTO:
-            if not model_module.is_backend_compatible():
-                raise ValueError(
-                    f"{arch} has no vLLM implementation and the Transformers "
-                    "implementation is not compatible with vLLM. Try setting "
-                    "VLLM_USE_V1=0.")
-            logger.warning(
-                "%s has no vLLM implementation, falling back to Transformers "
-                "implementation. Some features may not be supported and "
-                "performance may not be optimal.", arch)
-            architectures[i] = "TransformersForCausalLM"
+
+        if not model_module.is_backend_compatible():
+            raise ValueError(
+                f"The Transformers implementation of '{arch}' is not "
+                "compatible with vLLM.")
+
+        architectures[i] = model_config._get_transformers_backend_cls()
     return architectures
 
 
@@ -237,8 +239,9 @@ def get_model_architecture(
     ]
 
     vllm_supported_archs = ModelRegistry.get_supported_archs()
-    vllm_not_supported = not any(arch in vllm_supported_archs
-                                 for arch in architectures)
+    is_supported = lambda arch: (arch in vllm_supported_archs and arch not in
+                                 _TRANSFORMERS_MODELS)
+    vllm_not_supported = not any(is_supported(arch) for arch in architectures)
 
     if vllm_not_supported:
         # try automatic conversion in adapters.py
@@ -259,7 +262,7 @@ def get_model_architecture(
             break
 
     if (model_config.model_impl == ModelImpl.TRANSFORMERS or
-            model_config.model_impl != ModelImpl.VLLM and vllm_not_supported):
+            model_config.model_impl == ModelImpl.AUTO and vllm_not_supported):
         architectures = resolve_transformers_arch(model_config, architectures)
         logger.debug_once("Resolve transformers arch %s", str(architectures))
     elif (model_config.quantization is not None

vllm/model_executor/models/registry.py

@@ -252,6 +252,7 @@
 }
 
 _TRANSFORMERS_MODELS = {
+    "TransformersForMultimodalLM": ("transformers", "TransformersForMultimodalLM"), # noqa: E501
     "TransformersForCausalLM": ("transformers", "TransformersForCausalLM"),
 }
 # yapf: enable
@@ -503,9 +504,6 @@ def _normalize_archs(
             if causal_lm_arch in self.models:
                 normalized_arch.append(arch)
 
-        # make sure Transformers backend is put at the last as a fallback
-        if len(normalized_arch) != len(architectures):
-            normalized_arch.append("TransformersForCausalLM")
         return normalized_arch
 
     def inspect_model_cls(