Allow external CUDA streams to be set as current (#65914)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/65822.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65914
Reviewed By: dagitses
Differential Revision: D31389480
Pulled By: lw
fbshipit-source-id: 2b2f40e5452c5b2a0b9f0f705750d2aa9deb2ead
