[torchgen] Refactor torchgen.utils.FileManager to accept pathlib.Path (#150726)

This PR allows `FileManager` to accept `pathlib.Path` as arguments while keeping the original `str` path support.

This allows us to simplify the code such as:

1. `os.path.join(..., ...)` with `Path.__floordiv__(..., ...)`.

95a5958db4/torchgen/utils.py (L155)

95a5958db4/torchgen/utils.py (L176)

2. `os.path.basename(...)` with `Path(...).name`.
 95a5958db4/torchgen/utils.py (L161)

3. Manual file extension split with `Path(...).with_stem(new_stem)`

95a5958db4/torchgen/utils.py (L241-L256)

------

Pull Request resolved: https://github.com/pytorch/pytorch/pull/150726
Approved by: https://github.com/aorenste

torchgen/model.py

@@ -6,8 +6,9 @@ import re
 from dataclasses import dataclass
 from enum import auto, Enum
 from typing import Callable, Optional, TYPE_CHECKING
+from typing_extensions import assert_never
 
-from torchgen.utils import assert_never, NamespaceHelper, OrderedSet
+from torchgen.utils import NamespaceHelper, OrderedSet
 
 
 if TYPE_CHECKING: