improve ScalarQuantizer performance, ESPECIALLY on old GCC

[alexanderguzhva]

Introduces FAISS_ALWAYS_INLINE pragma directive and improves ScalarQuantizer performance with it.

Most of performance-critical methods for ScalarQuantizer are marked with this new directive, because a compiler (especially, an old one) may be unable to inline it properly. In some of my GCC experiments, such an inlining yields +50% queries per second in a search.

The corresponding Faiss PR is facebookresearch/faiss#3141.

/kind improvement

[mergify]

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1. If you're fixing a bug, label it as kind/bug.
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[cydrain]

/lgtm

[zhengbuqian]

/approve

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alexanderguzhva, zhengbuqian

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[yellow-shine]

/run-e2e

[cydrain]

/kind improvement