ANN_BENCH: common AnnBase::index_type

[achirkin]

Replace the size_t type in the AnnBase::search for the output neighbor indices with a common AnnBase::index_type.
This PR stops short of changing the behavior of the benchmarks, since it keeps using index_type = size_t.

The introduction of the new type has couple benefits:

• Makes the usage of the index_type more clear in the code, distinguishing it from the extents type, which is usually size_t as well.
• Makes it possible to quickly change the alias to uint32_t during development and experiments. This is needed to avoid calling extra linalg::map on the produced results when the algorithm output is not compatible with size_t.

As a small extra change, I've factored out common IVF-PQ - CAGRA-Q refinement code into a separate refine_helper function.

[tfeher]

Thanks Artem for the PR, it is nice to have an easy way to change index type in the benchmarks. ! It is great to have the helper function for refinement. LGTM.

[achirkin]

/merge