Set Python __slots__ on SWIG generated Python classes to avoid silent failures

[asadoughi]

Summary

Use Python slots to specify public member variables on C++ classes to avoid silent failures.

Reproduction instructions

index = faiss.index_factory(256, “OPQ64,IVF16384,PQ64”)
index.nprobe = 16

In this example, the silent failure misleads a user to believe they have set the nprobe parameter on the IVFPQ index. However, they have set a new Python object attribute nprobe on IndexPretransform which has no effect on the index search.

[mdouze]

This seems to be a "new-object" specific feature. Would it work with our "old-object" model?