`FaissIndex` to support multiple GPU and `custom_index`
Hi @rentruewang, thansk for reporting and for your PR!!! We should definitely support this.  @albertvillanova Great! :)
**Is your feature request related to a problem? Please describe.**
Currently, because `device` is of the type `int | None`, to leverage `faiss-gpu`'s multi-gpu support, you need to create a `custom_index`. However, if using a `custom_index` created by e.g. `faiss.index_cpu_to_all_gpus`, then `FaissIndex.save` does not work properly because it checks the device id (which is an int, so no multiple GPUs).

**Describe the solution you'd like**
I would like `FaissIndex` to support multiple GPUs, by passing in a list to `add_faiss_index`.

**Describe alternatives you've considered**
Alternatively, I would like it to at least provide a warning cause it wasn't the behavior that I expected.

**Additional context**
Relavent source code here:
 https://github.com/huggingface/datasets/blob/6ed6ac9448311930557810383d2cfd4fe6aae269/src/datasets/search.py#L340-L349

Device management needs changing to support multiple GPUs, probably by `isinstance` calls.

I can provide a PR if you like :)

Thanks for reading!
