feat(llms): support vLLM's OpenAI-compatible server (#9179)

This PR aims at supporting [vLLM's OpenAI-compatible server
feature](https://vllm.readthedocs.io/en/latest/getting_started/quickstart.html#openai-compatible-server),
i.e. allowing to call vLLM's LLMs like if they were OpenAI's.

I've also udpated the related notebook providing an example usage. At
the moment, vLLM only supports the `Completion` API.

libs/langchain/langchain/llms/__init__.py

@@ -80,7 +80,7 @@ from langchain.llms.textgen import TextGen
 from langchain.llms.titan_takeoff import TitanTakeoff
 from langchain.llms.tongyi import Tongyi
 from langchain.llms.vertexai import VertexAI
-from langchain.llms.vllm import VLLM
+from langchain.llms.vllm import VLLM, VLLMOpenAI
 from langchain.llms.writer import Writer
 from langchain.llms.xinference import Xinference
 
@@ -149,6 +149,7 @@ __all__ = [
     "Tongyi",
     "VertexAI",
     "VLLM",
+    "VLLMOpenAI",
     "Writer",
     "OctoAIEndpoint",
     "Xinference",
@@ -213,6 +214,7 @@ type_to_cls_dict: Dict[str, Type[BaseLLM]] = {
     "openllm": OpenLLM,
     "openllm_client": OpenLLM,
     "vllm": VLLM,
+    "vllm_openai": VLLMOpenAI,
     "writer": Writer,
     "xinference": Xinference,
 }

libs/langchain/langchain/llms/vllm.py

@@ -4,6 +4,7 @@ from pydantic import root_validator
 
 from langchain.callbacks.manager import CallbackManagerForLLMRun
 from langchain.llms.base import BaseLLM
+from langchain.llms.openai import BaseOpenAI
 from langchain.schema.output import Generation, LLMResult
 
 
@@ -127,3 +128,27 @@ class VLLM(BaseLLM):
     def _llm_type(self) -> str:
         """Return type of llm."""
         return "vllm"
+
+
+class VLLMOpenAI(BaseOpenAI):
+    """vLLM OpenAI-compatible API client"""
+
+    @property
+    def _invocation_params(self) -> Dict[str, Any]:
+        """Get the parameters used to invoke the model."""
+        openai_creds: Dict[str, Any] = {
+            "api_key": self.openai_api_key,
+            "api_base": self.openai_api_base,
+        }
+
+        return {
+            "model": self.model_name,
+            **openai_creds,
+            **self._default_params,
+            "logit_bias": None,
+        }
+
+    @property
+    def _llm_type(self) -> str:
+        """Return type of llm."""
+        return "vllm-openai"