mples/rag/use_own_knowledge_dataset.py
    >>> from transformers import RagRetriever

    >>> dataset_path = "path/to/my/dataset"  # dataset saved via *dataset.save_to_disk(...)*
    >>> index_path = "path/to/my/index.faiss"  # faiss index saved via *dataset.get_index("embeddings").save(...)*
    >>> retriever = RagRetriever.from_pretrained(
    ...     "facebook/dpr-ctx_encoder-single-nq-base",
    ...     index_name="custom",
    ...     passages_path=dataset_path,
    ...     index_path=index_path,
    ... )

    >>> # To load the legacy index built originally for Rag's paper
    >>> from transformers import RagRetriever

    >>> retriever = RagRetriever.from_pretrained("facebook/dpr-ctx_encoder-single-nq-base", index_name="legacy")
    ```NTc