Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/2539
Adding `clone_index()` for `IndexRefine` and `IndexRefineFlat` and several others index types.
https://github.com/facebookresearch/faiss/issues/2517
Note the change in `index_factory.cpp`, `RFlat` now constructs an `IndexRefineFlat`.
Reviewed By: mdouze
Differential Revision: D40511409
fbshipit-source-id: e55852eacb1f7d08be8c18005a82802939b4a6d9