Add CI scripts for Custom Build (#32316)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32316
### Summary
Since the Custom Build has been released in 1.4.0, it's time setup CI. To do that, we need
1. Add a python script to generate the yaml file
2. Add new build scripts to circle CI (arm64 only).
### Test Plan
- Don't break the current iOS CIs
Test Plan: Imported from OSS
Differential Revision: D19437362
Pulled By: xta0
fbshipit-source-id: 395e27a582c43663af88d11b1ef974a4687e672c
