def get_retriever(config, index):
    r"""Automatically select retriever class based on config's retrieval method

    Args:
        config (dict): configuration with 'retrieval_method' key

    Returns:
        Retriever: retriever instance
    """
    if config.retrieval_method == "bm25":
        raise NotImplementedError
        return BM25Retriever(config)
    else:
        return DenseRetriever(config, index)
Automatically select retriever class based on config's retrieval method

    Args:
        config (dict): configuration with 'retrieval_method' key

    Returns:
        Retriever: retriever instance
    
verl/workers/retriever_workers.py