MMR search for a query string.
:param query: The query string to search for. :type query: str :param k: Number of most similar documents to return. :type k: int :param fetch_k: Candidate pool size before MMR reranking. :type fetch_k: int :param lambda_mult: Diversity vs. relevance trade-off parameter. :type lambda_mult: float
See :meth:similarity_search for supported kwargs.
:return: Top-k documents. :rtype: list[Document]
