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

This reverts commit 9e1f3ecaa710785a1ab03c6ad5093a5566d6c5e5.

Reverted https://github.com/pytorch/pytorch/pull/129374 on behalf of https://github.com/huydhn due to Sorry for reverting your change but it is still failing with the same error ([comment](https://github.com/pytorch/pytorch/pull/129374#issuecomment-2197801405))

tools/setup_helpers/generate_code.py

@@ -1,25 +1,23 @@
 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: Path,
+    gen_dir: pathlib.Path,
     native_functions_path: Optional[str] = None,
     tags_path: Optional[str] = None,
     install_dir: Optional[str] = None,
@@ -30,7 +28,6 @@ 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
@@ -42,7 +39,7 @@ 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(Path(__file__).parent.parent / "autograd")
+    autograd_dir = os.fspath(pathlib.Path(__file__).parent.parent / "autograd")
 
     if subset == "pybindings" or not subset:
         gen_autograd_python(
@@ -109,9 +106,8 @@ def get_selector(
     operators_yaml_path: Optional[str],
 ) -> Any:
     # cwrap depends on pyyaml, so we can't import it earlier
-    REPO_ROOT = Path(__file__).absolute().parents[2]
-    sys.path.insert(0, str(REPO_ROOT))
-
+    root = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+    sys.path.insert(0, root)
     from torchgen.selective_build.selector import SelectiveBuilder
 
     assert not (
@@ -135,8 +131,8 @@ def main() -> None:
     parser.add_argument("--tags-path")
     parser.add_argument(
         "--gen-dir",
-        type=Path,
-        default=Path("."),
+        type=pathlib.Path,
+        default=pathlib.Path("."),
         help="Root directory where to install files. Defaults to the current working directory.",
     )
     parser.add_argument(