langchain/memory/token_buffer.py/ConversationTokenBufferMemory/load_memory_variables
class ConversationTokenBufferMemory:    def load_memory_variables(self, inputs: Dict[str, Any]) -> Dict[str, Any]:
        """Return history buffer."""
        buffer: Any = self.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}