This PR adds enforcement of testing header only APIs.
The benefit of torch/header_only_apis.txt is twofold:
1) this gives us a clear view of what we expect to be header only
2) this allows us to enforce testing
The enforcement added in this PR is very basic--we literally string match that a symbol in `torch/header_only_apis.txt` is in a cpp test. This is meant to be a first step in verifying our APIs are properly tested and can get fancier over time. For now, I've added myself as a codeowner to learn what to look out for in terms of proper tests. Over time, I anticipate we can automate more steps, but right now let's just get something out the door.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/153635
Approved by: https://github.com/albanD
ghstack dependencies: #153965
