[iOS] Add watchOS support (#33318)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33318
### Summary
Recently, we have a [discussion](https://discuss.pytorch.org/t/libtorch-on-watchos/69073/14) in the forum about watchOS. This PR adds the support for building watchOS libraries.
### Test Plan
- `BUILD_PYTORCH_MOBILE=1 IOS_PLATFORM=WATCHOS ./scripts/build_ios.sh`
Test Plan: Imported from OSS
Differential Revision: D19896534
Pulled By: xta0
fbshipit-source-id: 7b9286475e895d9fefd998246e7090ac92c4c9b6
