[doc] explaining the integration with huggingface (#10173)

Signed-off-by: youkaichao <youkaichao@gmail.com>

docs/source/design/huggingface_integration.rst

+Integration with HuggingFace
+===================================
+
+This document describes how vLLM integrates with HuggingFace libraries. We will explain step by step what happens under the hood when we run ``vllm serve``.
+
+Let's say we want to serve the popular QWen model by running ``vllm serve Qwen/Qwen2-7B``.
+
+1. The ``model`` argument is ``Qwen/Qwen2-7B``. vLLM will first try to locate the config file ``config.json`` using this argument. See the `code snippet <https://github.com/vllm-project/vllm/blob/10b67d865d92e376956345becafc249d4c3c0ab7/vllm/transformers_utils/config.py#L75>`__ for the implementation.
+
+   - If the ``model`` argument is a local path, vLLM will directly read the config file from the path.
+
+   - Otherwise, vLLM will try to read the config from the HuggingFace cache. See `their website <https://huggingface.co/docs/huggingface_hub/en/package_reference/environment_variables#hfhome>`__ for more information on how the HuggingFace cache works. Here, we can also use the argument ``--revision`` to specify the revision of the model in the cache.
+
+   - If neither of the above works, vLLM will download the config file from the HuggingFace model hub, using the ``model`` argument as the model name, the ``--revision`` argument as the revision, and the environment variable ``HF_TOKEN`` as the token to access the model hub. In our case, vLLM will download the `config.json <https://huggingface.co/Qwen/Qwen2-7B/blob/main/config.json>`__ file.
+
+2. After obtaining the config file, vLLM will load the config into a dictionary. It first `inspects <https://github.com/vllm-project/vllm/blob/10b67d865d92e376956345becafc249d4c3c0ab7/vllm/transformers_utils/config.py#L189>`__ the ``model_type`` field in the config to determine the model type and config class to use. There are some ``model_type`` values that vLLM directly supports; see `here <https://github.com/vllm-project/vllm/blob/10b67d865d92e376956345becafc249d4c3c0ab7/vllm/transformers_utils/config.py#L48>`__ for the list. If the ``model_type`` is not in the list, vLLM will use `AutoConfig.from_pretrained <https://huggingface.co/docs/transformers/en/model_doc/auto#transformers.AutoConfig.from_pretrained>`__ to load the config class, with ``model``, ``--revision``, and ``--trust_remote_code`` as the arguments.
+
+   - HuggingFace also has its own logic to determine the config class to use. It will again use the ``model_type`` field to search for the class name in the transformers library; see `here <https://github.com/huggingface/transformers/tree/main/src/transformers/models>`__ for the list of supported models. If the ``model_type`` is not found, HuggingFace will use the ``auto_map`` field from the config JSON file to determine the class name. Specifically, it is the ``AutoConfig`` field under ``auto_map``. See `DeepSeek <https://huggingface.co/deepseek-ai/DeepSeek-V2.5/blob/main/config.json>`__ for an example.
+
+   - The ``AutoConfig`` field under ``auto_map`` points to a module path in the model's repository. To create the config class, HuggingFace will import the module and use the ``from_pretrained`` method to load the config class. This can generally cause arbitrary code execution, so it is only executed when ``--trust_remote_code`` is enabled.
+
+3. After obtaining the config object, vLLM applies some historical patches to the config object. These are mostly related to RoPE configuration; see `here <https://github.com/vllm-project/vllm/blob/127c07480ecea15e4c2990820c457807ff78a057/vllm/transformers_utils/config.py#L244>`__ for the implementation.
+
+4. The config object is `attached <https://github.com/vllm-project/vllm/blob/10b67d865d92e376956345becafc249d4c3c0ab7/vllm/config.py#L195>`__ as the ``hf_config`` field to vLLM's ``model_config`` object.
+
+5. After vLLM obtains the config object, it will use the ``architectures`` field to determine the model class to initialize. For ``Qwen/Qwen2-7B``, the ``architectures`` field is ``["Qwen2ForCausalLM"]``. vLLM maintains the mapping from architecture name to model class in `its registry <https://github.com/vllm-project/vllm/blob/127c07480ecea15e4c2990820c457807ff78a057/vllm/model_executor/models/registry.py#L80>`__. If the architecture name is not found in the registry, it means this model architecture is not supported by vLLM.
+
+6. Finally, we reach the model class we want to initialize, i.e., the ``Qwen2ForCausalLM`` class in `vLLM's code <https://github.com/vllm-project/vllm/blob/127c07480ecea15e4c2990820c457807ff78a057/vllm/model_executor/models/qwen2.py#L364>`__. This class will initialize itself depending on various configs.
+
+Beyond that, there are two more things vLLM depends on HuggingFace for.
+
+1. **Tokenizer**: vLLM uses the tokenizer from HuggingFace to tokenize the input text. The tokenizer is loaded using `AutoTokenizer.from_pretrained <https://huggingface.co/docs/transformers/en/model_doc/auto#transformers.AutoTokenizer.from_pretrained>`__ with the ``model`` argument as the model name and the ``--revision`` argument as the revision. It is also possible to use a tokenizer from another model by specifying the ``--tokenizer`` argument in the ``vllm serve`` command. Other relevant arguments are ``--tokenizer-revision`` and ``--tokenizer-mode``. Please check HuggingFace's documentation for the meaning of these arguments. This part of the logic can be found in the `get_tokenizer <https://github.com/vllm-project/vllm/blob/127c07480ecea15e4c2990820c457807ff78a057/vllm/transformers_utils/tokenizer.py#L87>`__ function. After obtaining the tokenizer, notably, vLLM will cache some expensive attributes of the tokenizer in `get_cached_tokenizer <https://github.com/vllm-project/vllm/blob/127c07480ecea15e4c2990820c457807ff78a057/vllm/transformers_utils/tokenizer.py#L24>`__.
+
+2. **Model weight**: vLLM downloads the model weight from the HuggingFace model hub using the ``model`` argument as the model name and the ``--revision`` argument as the revision. vLLM provides the argument ``--load-format`` to control what files to download from the model hub. By default, it will try to load the weights in the safetensors format and fall back to the PyTorch bin format if the safetensors format is not available. We can also pass ``--load-format dummy`` to skip downloading the weights.
+
+   - It is recommended to use the safetensors format, as it is efficient for loading in distributed inference and also safe from arbitrary code execution. See the `documentation <https://huggingface.co/docs/safetensors/en/index>`__ for more information on the safetensors format.
+
+This completes the integration between vLLM and HuggingFace.
+
+In summary, vLLM reads the config file ``config.json``, tokenizer, and model weight from the HuggingFace model hub or a local directory. It uses the config class from either vLLM, HuggingFace transformers, or loads the config class from the model's repository.

docs/source/index.rst

@@ -159,6 +159,7 @@ Documentation
    design/input_processing/model_inputs_index
    design/kernel/paged_attention
    design/multimodal/multimodal_index
+   design/huggingface_integration
 
 .. Contributing: contributing to the vLLM project