Fix API that approximates the size of FAISS indexes

[CascadingRadium]

• We currently return 8 bytes for the faiss_IndexBinary_size and the
  faiss_Index_size APIs
• Although this could be correct, since we memory-map the FAISS indexes, this API is
  quite useless as it always returns 8 after performing a sizeof operation.
• When estimating the amount of memory a FAISS index would use on the GPU for throttling,
  this API cannot be used.
• Fix this issue by modifying the API behavior to return the approximate amount of memory
  it would require if we bring the entire index to the memory, which would be useful in
  approximating how much memory must be available in the GPU to
  consider the FAISS index to be eligible for cloning.

[Copilot]

Pull request overview

This PR updates the FAISS C “_ex” APIs for index sizing to return a more useful (approximate) in-memory byte estimate instead of effectively returning the pointer size, with the goal of supporting GPU-memory eligibility/throttling decisions when cloning indexes.

Changes:

• Changes faiss_Index_size / faiss_IndexBinary_size to return an error code (int) and write the size to an out-parameter (size_t*).
• Implements a new size estimate based on ntotal * code_size plus extra IVF-related overhead (centroids + stored IDs).
• Updates/expands header documentation and adds IVF includes to enable IVF-specific handling.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 8 comments.

File	Description
c_api/Index_c_ex.h	Updates C API signature + documentation for faiss_Index_size.
c_api/Index_c_ex.cpp	Implements the new size estimation logic for float indexes (including IVF path).
c_api/IndexBinary_c_ex.h	Updates C API signature + documentation for faiss_IndexBinary_size.
c_api/IndexBinary_c_ex.cpp	Implements the new size estimation logic for binary indexes (including IVF path).

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[Thejas-bhat]

looks good but can you please add the MB for it?