Fix the TestApp (#29247)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29247
### Summary
If you run the TestApp using Cocoapods, you'll likely run into an error due to the lack of `config.json` in the main bundle. This PR fixes this crash and updates the README as well.
### Test Plan
- Don't break CIs
Test Plan: Imported from OSS
Differential Revision: D18339047
Pulled By: xta0
fbshipit-source-id: 244cf1ca8729c7ac918258d4eff14d34363e8389
