Enhance build_ios.sh to be consistent with build_android.sh (#18564)
Summary:
1, Enhance build_ios.sh to be consistent with build_android.sh;
2, Add docs for build_ios.sh.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18564
Differential Revision: D14680752
Pulled By: soumith
fbshipit-source-id: 6d2667ed8a3c85a057a522838f5d0461dd4788cf
