langchain/callbacks/streamlit.py/StreamlitCallbackHandler/on_llm_new_token
class StreamlitCallbackHandler:    def on_llm_new_token(self, token: str, **kwargs: Any) -> None:
        """Run on new LLM token. Only available when streaming is enabled."""
        self.tokens_stream += token
        self.tokens_area.write(self.tokens_stream)