Make HIPStream also masquerade as CUDA. (#17469)

Make HIPStream also masquerade as CUDA. (#17469)

Summary:
HIPGuard interfaces that interacted with HIPStream were previously
totally busted (because the streams had the wrong device type).
This fixes it, following along the same lines of MasqueardingAsCUDA.

Along the way I beefed up the explanatory comment.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>
cc jithunnair-amd iotamudelta bddppq
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17469

Differential Revision: D14243396

Pulled By: ezyang

fbshipit-source-id: 972455753a62f8584ba9ab194f9c785db7bb9bde