langchain/callbacks/streamlit.py/StreamlitCallbackHandler/on_llm_start
class StreamlitCallbackHandler:    def on_llm_start(
        self, serialized: Dict[str, Any], prompts: List[str], **kwargs: Any
    ) -> None:
        """Print out the prompts."""
        st.write("Prompts after formatting:")
        for prompt in prompts:
            st.write(prompt)