Move check_label ci to mergebot (#92309)
Fixes #88098
### What Changed
* Moved `check_label.py` logic into `trymerge.py`
* Refactored relevant unittests
* ~~Dropped~~ Refactored `check_label.py` ci job
### Tests
`python .github/scripts/test_trymerge.py`
`python .github/scripts/test_check_labels.py`
`make lint & lintrunner -a`
### Notes to reviewers
This PR replaces the [original PR](https://github.com/pytorch/pytorch/pull/92225) to workaround the sticky EasyCLA failure mark on its first commit.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92309
Approved by: https://github.com/ZainRizvi
