[iOS] enable Metal in the nightly build (#62855)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/62855
Test Plan: Test on Private Pod with the [HelloWorld](https://fburl.com/3hiwkkhm) demo
Reviewed By: xta0
Differential Revision: D30174151
Pulled By: hanton
fbshipit-source-id: 22cd8663ac239811bf8ed1c3b6301460d798dbfa
