langchain/memory/summary.py/ConversationSummaryMemory/load_memory_variables
class ConversationSummaryMemory:    def load_memory_variables(self, inputs: Dict[str, Any]) -> Dict[str, Any]:
        """Return history buffer."""
        if self.return_messages:
            buffer: Any = [self.summary_message_cls(content=self.buffer)]
        else:
            buffer = self.buffer
        return {self.memory_key: buffer}