Replace a platform.system() check with sys.platform (#51766)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51766
Check if we are on Windows using `sys.platform` rather than
`platform.system()`. Even though `platform.system()` is more modern, it
has a few downsides: this performs a runtime check of the platform type,
which has non-zero overhead. On Linux it actually executes the separate
`/bin/uname` process. On the other hand `sys.platform` is determined
when the Python interpreter is compiled, so this is a simple hard-coded
string.
Because it is a runtime check, `platform.system()` checks also cannot be
analyzed by static type checkers like Pyre and Mypy. These type
checkers do understand `sys.platform` checks, and can correctly avoid
complaining about code paths that use platform-specific modules and
functions. e.g., they can avoid complaining about `ctypes.WinDLL` not
existing on Linux if its use is guarded by a `sys.platform` check.
ghstack-source-id: 121107705
Test Plan: Ran tests on Linux, and will check CI test results.
Reviewed By: mraway
Differential Revision: D26271724
Pulled By: simpkins
fbshipit-source-id: b86e427e4ceec0324464ba4bc88b95d5813172d0
