Allow disabling cosine distance from DeezyMatch to speed up retrieval

[mcollardanuy]

DeezyMatch issue: Living-with-machines/DeezyMatch#131

Cosine distance is always computed. If we're using faiss, we don't need to compute cosine distance, and would probably speed up candidate selection considerably.

The only thing we need to take into account is that we'll need the score we return in candidate ranking (ranking.py), because we use "cosine_dist", and we'd need to use "faiss" instead.