Fixes mem_get_info when querying on a device other than the current device (#69640)
Summary:
Also fixes the documentation failing to appear and adds a test to validate that op works with multiple devices properly.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69640
Reviewed By: ngimel
Differential Revision: D32965391
Pulled By: mruberry
fbshipit-source-id: 4fe502809b353464da8edf62d92ca9863804f08e
