langchain/callbacks/streaming_aiter.py/AsyncIteratorCallbackHandler/on_llm_start
class AsyncIteratorCallbackHandler:    async def on_llm_start(
        self, serialized: Dict[str, Any], prompts: List[str], **kwargs: Any
    ) -> None:
        # If two calls are made in a row, this resets the state
        self.done.clear()