Only label checks against pull requests (#87488)
When a commit is triggered via any mechanism other than a pull request, there will not be a PR to check labels for.
The job will fail with the error:
```
2022-10-21T17:50:53.2938592Z + python3 .github/scripts/check_labels.py ''
2022-10-21T17:50:53.4758863Z usage: Check PR labels [-h] pr_num
2022-10-21T17:50:53.4759337Z Check PR labels: error: argument pr_num: invalid int value: ''
```
Instead, we should limit the workflow to only run on pull requests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87488
Approved by: https://github.com/huydhn
