[Core] Registry for processing model inputs (#5214)

Co-authored-by: ywang96 <ywang@roblox.com>

docs/source/dev/input_processing/model_inputs_index.rst

@@ -0,0 +1,39 @@
+.. _input_processing:
+
+Input Processing
+================
+
+.. currentmodule:: vllm.inputs
+
+vLLM provides a mechanism for defining input processors for each model so that the inputs are processed
+in :class:`~vllm.LLMEngine` before they are passed to model executors. 
+
+Currently, this mechanism is only utilized in **multi-modal models** for preprocessing multi-modal input 
+data in addition to input prompt, but it can be extended to text-only language models when needed.
+
+Guides
+++++++
+
+.. toctree::
+   :maxdepth: 1
+
+   input_processing_pipeline
+
+Module Contents
++++++++++++++++
+
+LLM Engine Inputs
+-----------------
+
+.. autoclass:: vllm.inputs.LLMInputs
+    :members:
+    :show-inheritance:
+
+Registry
+--------
+
+.. autodata:: vllm.inputs.INPUT_REGISTRY
+
+.. automodule:: vllm.inputs.registry
+    :members:
+    :show-inheritance: