[RELAND] Adds torch.cuda.is_current_stream_capturing (#77789)
Resubmit of https://github.com/pytorch/pytorch/pull/77673, which was reverted due to Windows test failures: https://github.com/pytorch/pytorch/pull/77673#issuecomment-1130425845.
I suspect these failures happened because I don't explicitly set a side stream for graph capture in the new test.
Not setting a side stream explicitly is alright on Linux because cuda tests implicitly use a side stream.
I think Windows cuda tests implicitly use the default stream, breaking capture and leaving the backend in a bad state.
Other graphs tests explicitly set side streams and don't error in Windows builds, so i'm 95% sure doing the same for the new test will work.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77789
Approved by: https://github.com/ezyang
