Add support for other pytorch device types, including MPS

[adamobeng]

Fixes (#1441)

Change list

1. Add command line arguments --device_type and --device_ids which allow torch backend and device ordinals to be specified
2. Make code specific to GPUs/cuda device-agnostic (in particular by using a list of torch devices rather than GPU ids)
3. Maintain support for --gpu_ids argument with some special logic (it would be cleaner but non-backwards compatible to remove it)
4. Add some tests of the argument parsing

Testing

• Unit tests pass
• Results generated with python train.py --dataroot ./datasets/maps --name maps --model pix2pix --direction AtoB --device_type mps seem reasonable.
• Suggestions on more rigorous testing are welcomed!

NB: On my specific setup, loading a model trained with MPS fails with RuntimeError: don't know how to restore data location of torch.storage._UntypedStorage (tagged with mps:0), but it seems like this is a known and intermittent issue.