Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/2633
The core idea.
Instead of putting every element of the dataset into MaxHeap, split the dataset into buckets and let every bucket track elements min-1, min-2 or min-3 distances.
Applied to ResidualQuantizer class for vector codec purposes.
An example
```
rq.approx_topk_mode = faiss.APPROX_TOPK_BUCKETS_B16_D2
```
Reviewed By: mdouze
Differential Revision: D42044398
fbshipit-source-id: 43169026476650442806a31d1c1aa2d5d5028e65
Alexandr Guzhva