Cast query embeddings to fp32 before faiss search

[sahel-sh]

Currently the corpus embeddings are casted to fp32 before faiss search[1] but queries are not[2]. Even though --fp16 is supported for query encoders, as well[3].

[1] https://github.com/castorini/pyserini/blob/master/pyserini/index/faiss.py#L62
[2] https://github.com/castorini/pyserini/blob/master/pyserini/search/faiss/_searcher.py#L202
[3] https://github.com/castorini/pyserini/blob/e9b559c32c10893ae61c12a0c1e9ee2b264a2e41/pyserini/search/faiss/main.py#L294

This is a one line fix, but it might require an audit of all the 2crs using --fp16 for search.