[Feature] Add min_nprobe to get_optimal_hyperparameters function

[victor-paltz]

Some IVF KNN indices built by autofaiss have bad recall scores due to the unreachable speed constraints given in the requirements.

A way to guarantee decent recall scores is to force a minimal value to nprobe for IVF-like indices (it was already the case for IVF_HNSW indices with a min_ef_search parameter).

This commit makes it possible to set a custom value for min_nprobe

[rom1504]

There is a merge conflict here