Add PR jobs for iOS builds (#25840)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25840
### Summary:
The CI jobs for iOS builds are missing, this PR creates a workflow which will two PR jobs:
- pytorch_ios_10_2_1_x86_64_build
- pytorch_ios_10_2_1_arm64_build
### Note:
Those two jobs will not store any artifacts nor upload any binary files, which will be done in the next PR.
Test Plan:
- The jobs can be triggered by any PR.
- The jobs can be run successfully.
Differential Revision: D17255504
Pulled By: xta0
fbshipit-source-id: 5c56e85c7ccf6339a3e0ffd11eedd925f137adc8
