Consider exposing internal stats and metrics in C++/Python code

[alexanderguzhva]

Summary

Certain Faiss algorithms maintain internal statistics during the search or other functions. It helps if these statistics can be visible in outside world. For example, one could add extra pointer-based fields SearchParameters-derived classes that would be populated in a corresponding ::search() methods.

Such as

struct SearchParametersHNSW : SearchParameters {
    HNSWStats* output_stats = nullptr;
};

Faiss version:
master branch

Installed from:

Faiss compilation options: