Fix nullable-to-nonnull-conversion warnings (#106232)
Summary:
Cleaning up some build warnings when Wnullable-to-nonnull-conversion is enabled.
Changelog: Fixes nullability warnings from `MetalContext.mm`.
Test Plan:
```
buck build fbsource//fbobjc/mode/iphonesimulator //fbobjc/Apps/MSQRD/MSQRDPlayer:ARStudioPlayer
```
Differential Revision: D47886793
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106232
Approved by: https://github.com/drisspg
