langchain/memory/motorhead_memory.py/MotorheadMemory/load_memory_variables
class MotorheadMemory:    def load_memory_variables(self, values: Dict[str, Any]) -> Dict[str, Any]:
        if self.return_messages:
            return {self.memory_key: self.chat_memory.messages}
        else:
            return {self.memory_key: get_buffer_string(self.chat_memory.messages)}