Initialize the backend.

@param relevance_threshold: The relevance threshold for the MMR query engine. 0-1 (default: 0.5) The higher the threshold, the stricter the document relevance requirement. Increasing the threshold increases the relevance, but also decreases the chance of finding an answer.

Query the index with the user input.

Returns a tuple comprising (a) the response dicts and (b) the response object, if any.