Decouple DataParallel/DistributedDataParallel from CUDA (#38454)
Summary:
Decouple DataParallel/DistributedDataParallel from CUDA to support more device types.
- Move torch/cuda/comm.py to torch/nn/parallel/comm.py with minor changes for common devices support. Torch.cuda.comm is kept as is for backward compatibility
- Provide common APIs to arbitrary device types without changing existing CUDA APIs in torch.cuda space.
- Replace the torch.cuda calls in DataParellel/DistributedDataParallel with the new APIs.
Related RFC: [https://github.com/pytorch/pytorch/issues/36160](https://github.com/pytorch/pytorch/issues/36160)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38454
Differential Revision: D22051557
Pulled By: mrshenli
fbshipit-source-id: 7842dad0e5d3ca0f6fb760bda49182dcf6653af8
