[Feature Request] Batch Encoding of Queries #2270

In https://github.com/castorini/pyserini/blob/master/pyserini/encode/_auto.py
though AutoDocumentEncoder allows batch encoding, but AutoQueryEncoder encodes one query at a time.

This forces FaissSearcher # batch_search function to encode one query at a time

def _enc(q):
    if isinstance(q, dict):
        return self.query_encoder.encode(**q)
    assert isinstance(q, str)
    return self.query_encoder.encode(q)
q_embs = [_enc(q) for q in queries]

Allowing batch encoding for queries would improve performance of batch_search