langchain/memory/summary_buffer.py/ConversationSummaryBufferMemory/load_memory_variables
class ConversationSummaryBufferMemory:    def load_memory_variables(self, inputs: Dict[str, Any]) -> Dict[str, Any]:
        """Return history buffer."""
        buffer = self.buffer
        if self.moving_summary_buffer != "":
            first_messages: List[BaseMessage] = [
                self.summary_message_cls(content=self.moving_summary_buffer)
            ]
            buffer = first_messages + buffer
        if self.return_messages:
            final_buffer: Any = buffer
        else:
            final_buffer = get_buffer_string(
                buffer, human_prefix=self.human_prefix, ai_prefix=self.ai_prefix
            )
        return {self.memory_key: final_buffer}