langchain/callbacks/base.py/AsyncCallbackHandler/on_llm_start
class AsyncCallbackHandler:    async def on_llm_start(
        self,
        serialized: Dict[str, Any],
        prompts: List[str],
        *,
        run_id: UUID,
        parent_run_id: Optional[UUID] = None,
        **kwargs: Any,
    ) -> None:
        """Run when LLM starts running."""