[SVS] Implement memory-mapped I/O for Flat and static Vamana SVS indices

[rfsaliev]

This pull request adds support for memory-mapped I/O (mmap) loading of static SVS (Scalable Vector Search) indices in Faiss, enabling faster and more memory-efficient index loading. The main changes include implementing mmap-based deserialization for static Vamana and Flat indices, updating the index loading logic to use mmap when requested, and adding comprehensive tests for the new functionality.

Memory-mapped index loading support:

• Added map_to methods to IndexSVSVamana and IndexSVSFlat for loading static indices directly from memory-mapped files, including logic to manage memory mapping ownership and adjust file positions after mapping.

Index and file structure updates:

• Extended IndexSVSVamana and IndexSVSFlat classes to hold a reference to the mmap owner, ensuring the mapped memory remains valid for the lifetime of the index. Also added forward declarations and includes for mmap-related types.
• Modified reset methods to release memory mappings when the index is reset, preventing resource leaks.

Testing and interface improvements:

• Enhanced test utilities and added new tests to verify correct mmap loading for various static Vamana index types, including FP32, FP16, SQI8, LVQ4x4, and LeanVec4x4 in tests/test_svs.cpp

These changes collectively provide robust and efficient support for memory-mapped loading of static SVS indices, improving both performance and usability for large-scale vector search applications.

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[meta-codesync]

@mnorris11 has imported this pull request. If you are a Meta employee, you can view this in D108313487.