add convinence method to call chat model as an llm (#1604)

langchain/chat_models/base.py

@@ -12,6 +12,7 @@ from langchain.schema import (
     BaseMessage,
     ChatGeneration,
     ChatResult,
+    HumanMessage,
     LLMResult,
     PromptValue,
 )
@@ -116,6 +117,10 @@ class BaseChatModel(BaseLanguageModel, BaseModel, ABC):
     ) -> BaseMessage:
         return self._generate(messages, stop=stop).generations[0].message
 
+    def call_as_llm(self, message: str, stop: Optional[List[str]] = None) -> str:
+        result = self([HumanMessage(content=message)], stop=stop)
+        return result.content
+
 
 class SimpleChatModel(BaseChatModel):
     def _generate(