Summary:
## Description
The process of updating the codebook in LSQ may be unstable if the data is not zero-centering. This diff fixed it by using `double` instead of `float` during codebook updating. This would not affect the performance since the update process is quite fast.
Users could switch back to `float` mode by setting `update_codebooks_with_double = False`
## Changes
1. Support `double` during codebook updating.
2. Add a unit test.
3. Add `__init__.py` under `contrib/` to avoid warnings.
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/1975
Reviewed By: wickedfoo
Differential Revision: D29565632
Pulled By: mdouze
fbshipit-source-id: 932d7932ae9725c299cd83f87495542703ad6654
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