langchain/callbacks/streamlit.py/StreamlitCallbackHandler/on_chain_start
class StreamlitCallbackHandler:    def on_chain_start(
        self, serialized: Dict[str, Any], inputs: Dict[str, Any], **kwargs: Any
    ) -> None:
        """Print out that we are entering a chain."""
        class_name = serialized["name"]
        st.write(f"Entering new {class_name} chain...")