Simplify template instantiations

[tfeher]

This is a tracking issue to simplify the template instantiations in the src folder. The goal is to reduce code duplication and make it easier to apply changes.

There are code duplications for template instantiations. Compare:

• Macro factored out into separate include file https://github.com/rapidsai/cuvs/blob/branch-24.08/cpp/src/neighbors/detail/cagra/search_single_cta_float_uint32_dim1024_t32.cu
• Macro duplicated https://github.com/rapidsai/cuvs/blob/branch-24.08/cpp/src/neighbors/ivf_pq/detail/ivf_pq_search_float_int64_t.cu
• The directory structure of the src folder is more complex than ideal

Ref: #264 (comment)