Clarify that torch.device without an index will always represent the current device (#23468)
Summary:
Per discussion in https://github.com/pytorch/pytorch/issues/23448
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23468
Differential Revision: D16532950
Pulled By: soumith
fbshipit-source-id: 48c97060aaf55f1d7589afab42c6cd623d71a9a7
