[BE][Easy] use pathlib.Path instead of dirname / ".." / pardir (#129374)

Changes by apply order:

1. Replace all `".."` and `os.pardir` usage with `os.path.dirname(...)`.
2. Replace nested `os.path.dirname(os.path.dirname(...))` call with `str(Path(...).parent.parent)`.
3. Reorder `.absolute()` ~/ `.resolve()`~ and `.parent`: always resolve the path first.

    `.parent{...}.absolute()` -> `.absolute().parent{...}`

4. Replace chained `.parent x N` with `.parents[${N - 1}]`: the code is easier to read (see 5.)

    `.parent.parent.parent.parent` -> `.parents[3]`

5. ~Replace `.parents[${N - 1}]` with `.parents[${N} - 1]`: the code is easier to read and does not introduce any runtime overhead.~

    ~`.parents[3]` -> `.parents[4 - 1]`~

6. ~Replace `.parents[2 - 1]` with `.parent.parent`: because the code is shorter and easier to read.~

Pull Request resolved: https://github.com/pytorch/pytorch/pull/129374
Approved by: https://github.com/justinchuby, https://github.com/malfet

.github/scripts/test_gitutils.py

@@ -68,7 +68,7 @@ class TestRetriesDecorator(TestCase):
 
 class TestGitRepo(TestCase):
     def setUp(self) -> None:
-        repo_dir = BASE_DIR.parent.parent.absolute()
+        repo_dir = BASE_DIR.absolute().parent.parent
         if not (repo_dir / ".git").is_dir():
             raise SkipTest(
                 "Can't find git directory, make sure to run this test on real repo checkout"