langchain/experimental/generative_agents/memory.py/GenerativeAgentMemory/fetch_memories
class GenerativeAgentMemory:    def fetch_memories(
        self, observation: str, now: Optional[datetime] = None
    ) -> List[Document]:
        """Fetch related memories."""
        if now is not None:
            with mock_now(now):
                return self.memory_retriever.get_relevant_documents(observation)
        else:
            return self.memory_retriever.get_relevant_documents(observation)