[Misc] Split up pooling tasks (#10820)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

docs/source/models/pooling_models.rst

@@ -0,0 +1,99 @@
+.. _pooling_models:
+
+Pooling Models
+==============
+
+vLLM also supports pooling models, including embedding, reranking and reward models.
+
+In vLLM, pooling models implement the :class:`~vllm.model_executor.models.VllmModelForPooling` interface.
+These models use a :class:`~vllm.model_executor.layers.Pooler` to aggregate the final hidden states of the input
+before returning them.
+
+.. note::
+
+    We currently support pooling models primarily as a matter of convenience.
+    As shown in the :ref:`Compatibility Matrix <compatibility_matrix>`, most vLLM features are not applicable to
+    pooling models as they only work on the generation or decode stage, so performance may not improve as much.
+
+Offline Inference
+-----------------
+
+The :class:`~vllm.LLM` class provides various methods for offline inference.
+See :ref:`Engine Arguments <engine_args>` for a list of options when initializing the model.
+
+For pooling models, we support the following :code:`task` options:
+
+- Embedding (:code:`"embed"` / :code:`"embedding"`)
+- Classification (:code:`"classify"`)
+- Sentence Pair Scoring (:code:`"score"`)
+- Reward Modeling (:code:`"reward"`)
+
+The selected task determines the default :class:`~vllm.model_executor.layers.Pooler` that is used:
+
+- Embedding: Extract only the hidden states corresponding to the last token, and apply normalization.
+- Classification: Extract only the hidden states corresponding to the last token, and apply softmax.
+- Sentence Pair Scoring: Extract only the hidden states corresponding to the last token, and apply softmax.
+- Reward Modeling: Extract all of the hidden states and return them directly.
+
+When loading `Sentence Transformers <https://huggingface.co/sentence-transformers>`__ models,
+we attempt to override the default pooler based on its Sentence Transformers configuration file (:code:`modules.json`).
+
+You can customize the model's pooling method via the :code:`override_pooler_config` option,
+which takes priority over both the model's and Sentence Transformers's defaults.
+
+``LLM.encode``
+^^^^^^^^^^^^^^
+
+The :class:`~vllm.LLM.encode` method is available to all pooling models in vLLM.
+It returns the aggregated hidden states directly.
+
+.. code-block:: python
+
+    llm = LLM(model="intfloat/e5-mistral-7b-instruct", task="embed")
+    outputs = llm.encode("Hello, my name is")
+
+    outputs = model.encode(prompts)
+    for output in outputs:
+        embeddings = output.outputs.embedding
+        print(f"Prompt: {prompt!r}, Embeddings (size={len(embeddings)}: {embeddings!r}")
+
+A code example can be found in `examples/offline_inference_embedding.py <https://github.com/vllm-project/vllm/blob/main/examples/offline_inference_embedding.py>`_.
+
+``LLM.score``
+^^^^^^^^^^^^^
+
+The :class:`~vllm.LLM.score` method outputs similarity scores between sentence pairs.
+It is primarily designed for `cross-encoder models <https://www.sbert.net/examples/applications/cross-encoder/README.html>`__.
+These types of models serve as rerankers between candidate query-document pairs in RAG systems.
+
+.. note::
+
+    vLLM can only perform the model inference component (e.g. embedding, reranking) of RAG.
+    To handle RAG at a higher level, you should use integration frameworks such as `LangChain <https://github.com/langchain-ai/langchain>`_.
+
+You can use `these tests <https://github.com/vllm-project/vllm/blob/main/tests/models/embedding/language/test_scoring.py>`_ as reference.
+
+Online Inference
+----------------
+
+Our `OpenAI Compatible Server <../serving/openai_compatible_server>`__ can be used for online inference.
+Please click on the above link for more details on how to launch the server.
+
+Embeddings API
+^^^^^^^^^^^^^^
+
+Our Embeddings API is similar to ``LLM.encode``, accepting both text and :ref:`multi-modal inputs <multimodal_inputs>`.
+
+The text-only API is compatible with `OpenAI Embeddings API <https://platform.openai.com/docs/api-reference/embeddings>`__
+so that you can use OpenAI client to interact with it.
+A code example can be found in `examples/openai_embedding_client.py <https://github.com/vllm-project/vllm/blob/main/examples/openai_embedding_client.py>`_.
+
+The multi-modal API is an extension of the `OpenAI Embeddings API <https://platform.openai.com/docs/api-reference/embeddings>`__
+that incorporates `OpenAI Chat Completions API <https://platform.openai.com/docs/api-reference/chat>`__,
+so it is not part of the OpenAI standard. Please see :ref:`this page <multimodal_inputs>` for more details on how to use it.
+
+Score API
+^^^^^^^^^
+
+Our Score API is similar to ``LLM.score``.
+Please see `this page <../serving/openai_compatible_server.html#score-api-for-cross-encoder-models>`__ for more details on how to use it.