OpenAI functions dont work with async streaming... #6225 (#6226)

Related to this https://github.com/hwchase17/langchain/issues/6225

Just copied the implementation from `generate` function to `agenerate`
and tested it.

Didn't run any official tests thought

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Fixes #6225

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langchain/chat_models/openai.py

@@ -386,16 +386,27 @@ class ChatOpenAI(BaseChatModel):
             inner_completion = ""
             role = "assistant"
             params["stream"] = True
+            function_call: Optional[dict] = None
             async for stream_resp in await acompletion_with_retry(
                 self, messages=message_dicts, **params
             ):
                 role = stream_resp["choices"][0]["delta"].get("role", role)
                 token = stream_resp["choices"][0]["delta"].get("content", "")
-                inner_completion += token
+                inner_completion += token or ""
+                _function_call = stream_resp["choices"][0]["delta"].get("function_call")
+                if _function_call:
+                    if function_call is None:
+                        function_call = _function_call
+                    else:
+                        function_call["arguments"] += _function_call["arguments"]
                 if run_manager:
                     await run_manager.on_llm_new_token(token)
             message = _convert_dict_to_message(
-                {"content": inner_completion, "role": role}
+                {
+                    "content": inner_completion,
+                    "role": role,
+                    "function_call": function_call,
+                }
             )
             return ChatResult(generations=[ChatGeneration(message=message)])
         else: