Add default_stream() and enhance current_stream() (#16200)
Summary:
Closes #16156
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16200
Differential Revision: D13747455
Pulled By: mrshenli
fbshipit-source-id: 00c0d5f341c3ac7a757bdb4631a17e11fbc6d3ec
