Introduce `IS_LINUX` and `IS_MACOS` global vars (#69093)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/69093
Test Plan: Imported from OSS
Reviewed By: samdow
Differential Revision: D32730080
Pulled By: malfet
fbshipit-source-id: aa3f218d09814b4edd96b01c7b57b85fd58c47fc
