[Core] enable out-of-tree model register (#3871)

docs/source/models/adding_model.rst

@@ -21,6 +21,8 @@ This document provides a high-level guide on integrating a `HuggingFace Transfor
 Start by forking our `GitHub`_ repository and then :ref:`build it from source <build_from_source>`.
 This gives you the ability to modify the codebase and test your model.
 
+.. tip::
+    If you don't want to fork the repository and modify vLLM's codebase, please refer to the "Out-of-Tree Model Integration" section below.
 
 1. Bring your model code
 ------------------------
@@ -94,3 +96,28 @@ This method should load the weights from the HuggingFace's checkpoint file and a
 ----------------------
 
 Finally, include your :code:`*ForCausalLM` class in `vllm/model_executor/models/__init__.py <https://github.com/vllm-project/vllm/blob/main/vllm/model_executor/models/__init__.py>`_ and register it to the :code:`_MODEL_REGISTRY` in `vllm/model_executor/model_loader.py <https://github.com/vllm-project/vllm/blob/main/vllm/model_executor/model_loader.py>`_.
+
+6. Out-of-Tree Model Integration
+--------------------------------------------
+
+We also provide a way to integrate a model without modifying the vLLM codebase. Step 2, 3, 4 are still required, but you can skip step 1 and 5.
+
+Just add the following lines in your code:
+
+.. code-block:: python
+
+    from vllm import ModelRegistry
+    from your_code import YourModelForCausalLM
+    ModelRegistry.register_model("YourModelForCausalLM", YourModelForCausalLM)
+
+If you are running api server with `python -m vllm.entrypoints.openai.api_server args`, you can wrap the entrypoint with the following code:
+
+.. code-block:: python
+
+    from vllm import ModelRegistry
+    from your_code import YourModelForCausalLM
+    ModelRegistry.register_model("YourModelForCausalLM", YourModelForCausalLM)
+    import runpy
+    runpy.run_module('vllm.entrypoints.openai.api_server', run_name='__main__')
+
+Save the above code in a file and run it with `python your_file.py args`.