[LTC] Add some comments to BackendDevice() (#68156)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68156
[skip ci]
Test Plan: Imported from OSS
Reviewed By: wconstab
Differential Revision: D32346302
Pulled By: alanwaketan
fbshipit-source-id: 06de6afbe2f937511abce485b24cec0a85bfbe97
