add async default (#4701)

a spin on
https://github.com/hwchase17/langchain/pull/4300/files#diff-4f16071d58cd34fb3ec5cd5089e9dbd6fb06574c25c76b4d573827f8a2f48e96

langchain/chat_models/base.py

@@ -2,6 +2,7 @@ import asyncio
 import inspect
 import warnings
 from abc import ABC, abstractmethod
+from functools import partial
 from typing import Any, Dict, List, Mapping, Optional, Sequence
 
 from pydantic import Extra, Field, root_validator
@@ -239,3 +240,12 @@ class SimpleChatModel(BaseChatModel):
         run_manager: Optional[CallbackManagerForLLMRun] = None,
     ) -> str:
         """Simpler interface."""
+
+    async def _agenerate(
+        self,
+        messages: List[BaseMessage],
+        stop: Optional[List[str]] = None,
+        run_manager: Optional[AsyncCallbackManagerForLLMRun] = None,
+    ) -> ChatResult:
+        func = partial(self._generate, messages, stop=stop, run_manager=run_manager)
+        return await asyncio.get_event_loop().run_in_executor(None, func)