[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

test/mobile/test_bytecode.py

@@ -20,6 +20,7 @@ from torch.jit.mobile import (
 )
 from torch.testing._internal.common_utils import run_tests, TestCase
 
+
 pytorch_test_dir = Path(__file__).resolve().parents[1]
 
 # script_module_v4.ptl and script_module_v5.ptl source code