[FEA] faiss-gpu support

[lmeyerov]

Is your feature request related to a problem? Please describe.

We are getting some conda version conflicts with faiss-cpu, so switching to pip for now, but it'd probably be even better to support faiss-gpu when available, even if just single-gpu . It looks doable: https://github.com/facebookresearch/faiss/wiki/Running-on-GPUs#in-python-2 .

Describe the solution you'd like

Automatic opt-in to faiss-gpu when available

[lmeyerov]

cc @tanmoyio @silkspace

I'll try to test pip install faiss-cpu / #cleanup against the base image for now...