faiss-cpu faiss-gpu issue

I encountered this issue when I was trying to index colbert-ir/colbertv2.0:

`WARNING! You have a GPU available, but only `faiss-cpu` is currently installed.
This means that indexing will be slow. To make use of your GPU.
Please install `faiss-gpu` by running:
pip uninstall --y faiss-cpu & pip install faiss-gpu`

I then run the pip command to uninstall faiss-cpu and install faiss-gpu.
Then I got module not found:

` File "/WAVE/users/unix/jnian/.local/lib/python3.9/site-packages/ragatouille/models/colbert.py", line 313, in index
import faiss
ModuleNotFoundError: No module named 'faiss'`

Start by reproducing the ColBERT indexing flow described in the issue with the reported faiss-cpu and faiss-gpu installation commands. Inspect ragatouille/models/colbert.py around line 313 and verify the import behavior in that environment; done means GPU-backed indexing no longer fails with ModuleNotFoundError while the CPU-only warning remains accurate.

Written by the indexing model from the issue text.