Add a new job to support custom build (#32323)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32323
### Summary
Since we have released the custom build in 1.4.0, it's time to setup a CI for that. This PR adds a new iOS job to the iOS builds. To save time, It only runs the arm64 build.
### Test Plan
- Don't break any iOS jobs
- Custom Build works.
Test Plan: Imported from OSS
Differential Revision: D19451342
Pulled By: xta0
fbshipit-source-id: 9de305c004fc795710ecf01d436ef4792c07760c
