langchain/callbacks/manager.py/CallbackManager/on_chain_start
class CallbackManager:    def on_chain_start(
        self,
        serialized: Dict[str, Any],
        inputs: Dict[str, Any],
        run_id: Optional[UUID] = None,
        **kwargs: Any,
    ) -> CallbackManagerForChainRun:
        """Run when chain starts running."""
        if run_id is None:
            run_id = uuid4()

        _handle_event(
            self.handlers,
            "on_chain_start",
            "ignore_chain",
            serialized,
            inputs,
            run_id=run_id,
            parent_run_id=self.parent_run_id,
            **kwargs,
        )

        return CallbackManagerForChainRun(
            run_id, self.handlers, self.inheritable_handlers, self.parent_run_id
        )