Multi-GPU support for `FaissIndex`
Any love? Hi, any update?
Per #3716 , current implementation does not take into consideration that `faiss` can run on multiple GPUs.

In this commit, I provided multi-GPU support for `FaissIndex` by modifying the device management in `IndexableMixin.add_faiss_index` and `FaissIndex.load`.

Now users are able to pass in
1. a positive integer (as usual) to use 1 GPU
2. a negative integer `-1` to use all GPUs
3. a list of integers e.g. `[0, 1]` to run only on those GPUs
4. Of course, passing in nothing still runs on CPU.

This closes: #3716 