langchain/vectorstores/base.py/VectorStore/amax_marginal_relevance_search_by_vector
class VectorStore:    async def amax_marginal_relevance_search_by_vector(
        self,
        embedding: List[float],
        k: int = 4,
        fetch_k: int = 20,
        lambda_mult: float = 0.5,
        **kwargs: Any,
    ) -> List[Document]:
        """Return docs selected using the maximal marginal relevance."""
        raise NotImplementedError