langchain/callbacks/streaming_stdout.py/StreamingStdOutCallbackHandler/on_llm_new_token
class StreamingStdOutCallbackHandler:    def on_llm_new_token(self, token: str, **kwargs: Any) -> None:
        """Run on new LLM token. Only available when streaming is enabled."""
        sys.stdout.write(token)
        sys.stdout.flush()