Move check labels to separate workflow (#87999)
* moves check labels to separate workflow that is triggered on the usual pull_request triggers as well as labeled and unlabeled
* deletes comments when label is added
Fixes https://github.com/pytorch/test-infra/issues/978 and https://github.com/pytorch/pytorch/issues/87865
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87999
Approved by: https://github.com/huydhn
