[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

tools/setup_helpers/generate_code.py

@@ -1,23 +1,25 @@
 import argparse
 import os
-import pathlib
 import sys
+from pathlib import Path
 from typing import Any, cast, Optional
 
 import yaml
 
+
 try:
     # use faster C loader if available
     from yaml import CSafeLoader as YamlLoader
 except ImportError:
     from yaml import SafeLoader as YamlLoader  # type: ignore[assignment, misc]
 
+
 NATIVE_FUNCTIONS_PATH = "aten/src/ATen/native/native_functions.yaml"
 TAGS_PATH = "aten/src/ATen/native/tags.yaml"
 
 
 def generate_code(
-    gen_dir: pathlib.Path,
+    gen_dir: Path,
     native_functions_path: Optional[str] = None,
     tags_path: Optional[str] = None,
     install_dir: Optional[str] = None,
@@ -28,6 +30,7 @@ def generate_code(
 ) -> None:
     from tools.autograd.gen_annotated_fn_args import gen_annotated
     from tools.autograd.gen_autograd import gen_autograd, gen_autograd_python
+
     from torchgen.selective_build.selector import SelectiveBuilder
 
     # Build ATen based Variable classes
@@ -39,7 +42,7 @@ def generate_code(
     autograd_gen_dir = os.path.join(install_dir, "autograd", "generated")
     for d in (autograd_gen_dir, python_install_dir):
         os.makedirs(d, exist_ok=True)
-    autograd_dir = os.fspath(pathlib.Path(__file__).parent.parent / "autograd")
+    autograd_dir = os.fspath(Path(__file__).parent.parent / "autograd")
 
     if subset == "pybindings" or not subset:
         gen_autograd_python(
@@ -106,8 +109,9 @@ def get_selector(
     operators_yaml_path: Optional[str],
 ) -> Any:
     # cwrap depends on pyyaml, so we can't import it earlier
-    root = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
-    sys.path.insert(0, root)
+    REPO_ROOT = Path(__file__).absolute().parents[2]
+    sys.path.insert(0, str(REPO_ROOT))
+
     from torchgen.selective_build.selector import SelectiveBuilder
 
     assert not (
@@ -131,8 +135,8 @@ def main() -> None:
     parser.add_argument("--tags-path")
     parser.add_argument(
         "--gen-dir",
-        type=pathlib.Path,
-        default=pathlib.Path("."),
+        type=Path,
+        default=Path("."),
         help="Root directory where to install files. Defaults to the current working directory.",
     )
     parser.add_argument(