[FEA] Consolidate template parameters

[benfred]

We've had to add template parameters for both int64_t and int32 indices in the public api. We should consolidate these types to a single integral type where possible in the public api.

For functions like pairwise distances that benefit from working on int32 sized data, we should dispatch privately from int64_t to int32_t - and potentially tile the calculations for large inputs.

We should also examine consolidating the double and single precision float inputs : I believe that faiss only supports float32 data, and that might also be an option here