Output non-normalized distances in IVF-PQ and brute-force KNN

[Nyrio]

Solves point 9 of #711 (the observed errors were due to comparing distances at different scales).

This PR does two things:

• Changes the naive BF KNN to not normalize int when converting to float and simply cast, resulting in distances in the same scale as the output of IVF-Flat (and solving errors observed in tests).
• Changes IVF-PQ to output distances in the original scale and not the normalized scale, consistently with IVF-Flat.

[achirkin]

Thanks for the fix! I think we can make it a bit less complicated, see the couple comments

[Nyrio]

There are quite a few IVF-PQ cases where this PR gives worse results than before, I'll look into those.

[Nyrio]

Note: the worse test results that I observed in IVF-PQ are due to how raft::CompareApprox works: some mismatches were previously tolerated because the absolute error was lower than eps, which could only happen in the normalized scale but doesn't happen anymore in the original scale. So there is not much more we can do. Note that the tests are passing anyway.

Also, I am removing the "breaking" flag, since IVF-PQ was added in the same release.

[achirkin]

Thanks, the changes seem nice and non-intrusive to me. Just couple of tiny suggestions.

[tfeher]

Thanks Louis for the PR, it looks good to me!

[achirkin]

Thanks!

[cjnolet]

@gpucibot merge