Do not define C10_IOS on Mac (#37283)
Summary:
Because MacOS is not iOS
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37283
Test Plan: CI
Differential Revision: D21244398
Pulled By: malfet
fbshipit-source-id: b822e216e83887e2f2961b5c5384eaf749629f61
