[BE][Easy][5/19] enforce style for empty lines in import segments in tools/ and torchgen/ (#129756)

See https://github.com/pytorch/pytorch/pull/129751#issue-2380881501. Most changes are auto-generated by linter.

You can review these PRs via:

```bash
git diff --ignore-all-space --ignore-blank-lines HEAD~1
```

Pull Request resolved: https://github.com/pytorch/pytorch/pull/129756
Approved by: https://github.com/ezyang

tools/test/test_test_selections.py

@@ -7,6 +7,7 @@ import unittest
 from collections import defaultdict
 from pathlib import Path
 
+
 REPO_ROOT = Path(__file__).resolve().parent.parent.parent
 try:
     # using tools/ to optimize test run.