Replace caffe2::DeviceGuard with c10::cuda::CUDAGuard (#17623)

Replace caffe2::DeviceGuard with c10::cuda::CUDAGuard (#17623)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17623

Despite it's generic sounding name, caffe2::DeviceGuard actually
only worked on CUDA devices.  Rename it to something that more
clearly spells out its applicability.

I'm not sure if it's the right call, but in this patch I added
'using CUDAGuard = c10::cuda::CUDAGuard', as this seems to be more
in-line with how the Caffe2 codebase is currently written.  More
idiomatic c10 namespace style would be to say cuda::CUDAGuard.
Willing to change this if people shout.

This is a respin of D13156470 (#14284)

Reviewed By: dzhulgakov

Differential Revision: D14285504

fbshipit-source-id: 93b8ab938b064572b3b010c307e1261fde0fff3d