diff --git a/tools/autograd/gen_autograd_functions.py b/tools/autograd/gen_autograd_functions.py
index 69474e06d18e..cdc805d5a4b5 100644
--- a/tools/autograd/gen_autograd_functions.py
+++ b/tools/autograd/gen_autograd_functions.py
@@ -555,8 +555,7 @@ def gen_autograd_functions_lib(
             fname,
             lambda: {
                 "generated_comment": "@"
-                + f"generated from {fm.template_dir_for_comments()}/"
-                + fname,
+                + f"generated from {fm.template_dir_for_comments()}/{fname}",
                 "autograd_function_declarations": declarations,
                 "autograd_function_definitions": definitions,
             },
diff --git a/tools/autograd/gen_view_funcs.py b/tools/autograd/gen_view_funcs.py
index e6600106dca9..8cc8a2ffcecc 100644
--- a/tools/autograd/gen_view_funcs.py
+++ b/tools/autograd/gen_view_funcs.py
@@ -331,8 +331,7 @@ def gen_view_funcs(
             fname,
             lambda: {
                 "generated_comment": "@"
-                + f"generated from {fm.template_dir_for_comments()}/"
-                + fname,
+                + f"generated from {fm.template_dir_for_comments()}/{fname}",
                 "view_func_declarations": declarations,
                 "view_func_definitions": definitions,
                 "ops_headers": ops_headers,
diff --git a/torchgen/api/cpp.py b/torchgen/api/cpp.py
index 554bfa4a5c79..c619ec45d2f8 100644
--- a/torchgen/api/cpp.py
+++ b/torchgen/api/cpp.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from typing import TYPE_CHECKING
+from typing_extensions import assert_never
 
 from torchgen import local
 from torchgen.api.types import (
@@ -48,7 +49,6 @@ from torchgen.model import (
     TensorOptionsArguments,
     Type,
 )
-from torchgen.utils import assert_never
 
 
 if TYPE_CHECKING:
diff --git a/torchgen/api/dispatcher.py b/torchgen/api/dispatcher.py
index 4cc6186d7e0e..fcca7a60fec1 100644
--- a/torchgen/api/dispatcher.py
+++ b/torchgen/api/dispatcher.py
@@ -2,6 +2,7 @@ from __future__ import annotations
 
 import itertools
 from typing import TYPE_CHECKING
+from typing_extensions import assert_never
 
 from torchgen.api import cpp
 from torchgen.api.types import ArgName, Binding, CType, NamedCType
@@ -13,7 +14,7 @@ from torchgen.model import (
     TensorOptionsArguments,
     Type,
 )
-from torchgen.utils import assert_never, concatMap
+from torchgen.utils import concatMap
 
 
 if TYPE_CHECKING:
diff --git a/torchgen/api/native.py b/torchgen/api/native.py
index 82bc051a6832..632216704d2d 100644
--- a/torchgen/api/native.py
+++ b/torchgen/api/native.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from typing import TYPE_CHECKING
+from typing_extensions import assert_never
 
 from torchgen import local
 from torchgen.api import cpp
@@ -29,7 +30,6 @@ from torchgen.model import (
     TensorOptionsArguments,
     Type,
 )
-from torchgen.utils import assert_never
 
 
 if TYPE_CHECKING:
diff --git a/torchgen/api/structured.py b/torchgen/api/structured.py
index 93a72eb2b4a5..a0e14e5b69e6 100644
--- a/torchgen/api/structured.py
+++ b/torchgen/api/structured.py
@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from typing_extensions import assert_never
+
 from torchgen.api import cpp
 from torchgen.api.types import (
     ArgName,
@@ -30,7 +32,6 @@ from torchgen.model import (
     TensorOptionsArguments,
     Type,
 )
-from torchgen.utils import assert_never
 
 
 # This file describes the translation of JIT schema to the structured functions API.
diff --git a/torchgen/dest/register_dispatch_key.py b/torchgen/dest/register_dispatch_key.py
index 5b7feef83237..af3d4c9ca740 100644
--- a/torchgen/dest/register_dispatch_key.py
+++ b/torchgen/dest/register_dispatch_key.py
@@ -4,6 +4,7 @@ import itertools
 import textwrap
 from dataclasses import dataclass
 from typing import Literal, TYPE_CHECKING
+from typing_extensions import assert_never
 
 import torchgen.api.cpp as cpp
 import torchgen.api.meta as meta
@@ -36,7 +37,7 @@ from torchgen.model import (
     SchemaKind,
     TensorOptionsArguments,
 )
-from torchgen.utils import assert_never, mapMaybe, Target
+from torchgen.utils import mapMaybe, Target
 
 
 if TYPE_CHECKING:
diff --git a/torchgen/executorch/api/et_cpp.py b/torchgen/executorch/api/et_cpp.py
index 72b0551d029d..081a3d4ece1c 100644
--- a/torchgen/executorch/api/et_cpp.py
+++ b/torchgen/executorch/api/et_cpp.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from typing import TYPE_CHECKING
+from typing_extensions import assert_never
 
 from torchgen import local
 from torchgen.api.types import (
@@ -37,7 +38,6 @@ from torchgen.model import (
     TensorOptionsArguments,
     Type,
 )
-from torchgen.utils import assert_never
 
 
 if TYPE_CHECKING:
diff --git a/torchgen/executorch/model.py b/torchgen/executorch/model.py
index 6be7501ebead..310c5968ec0d 100644
--- a/torchgen/executorch/model.py
+++ b/torchgen/executorch/model.py
@@ -7,6 +7,7 @@ import itertools
 from collections import defaultdict, namedtuple
 from dataclasses import dataclass
 from enum import IntEnum
+from typing_extensions import assert_never
 
 from torchgen.model import (
     BackendIndex,
@@ -16,7 +17,6 @@ from torchgen.model import (
     NativeFunctionsGroup,
     OperatorName,
 )
-from torchgen.utils import assert_never
 
 
 KERNEL_KEY_VERSION = 1
diff --git a/torchgen/gen.py b/torchgen/gen.py
index 609d338887e6..1c27d32be727 100644
--- a/torchgen/gen.py
+++ b/torchgen/gen.py
@@ -9,6 +9,7 @@ from collections import defaultdict, namedtuple, OrderedDict
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Any, Callable, Literal, TYPE_CHECKING, TypeVar
+from typing_extensions import assert_never
 
 import yaml
 
@@ -84,7 +85,6 @@ from torchgen.native_function_generation import (
 )
 from torchgen.selective_build.selector import SelectiveBuilder
 from torchgen.utils import (
-    assert_never,
     concatMap,
     context,
     FileManager,
diff --git a/torchgen/model.py b/torchgen/model.py
index b6509b57748b..4715759d322e 100644
--- a/torchgen/model.py
+++ b/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:
diff --git a/torchgen/utils.py b/torchgen/utils.py
index 2d760a51145b..905d6fd0c0b6 100644
--- a/torchgen/utils.py
+++ b/torchgen/utils.py
@@ -11,7 +11,7 @@ from dataclasses import fields, is_dataclass
 from enum import auto, Enum
 from pathlib import Path
 from typing import Any, Callable, Generic, Literal, NoReturn, TYPE_CHECKING, TypeVar
-from typing_extensions import Self
+from typing_extensions import assert_never, deprecated, Self
 
 from torchgen.code_template import CodeTemplate
 
@@ -21,7 +21,8 @@ if TYPE_CHECKING:
     from collections.abc import Iterable, Iterator, Sequence
 
 
-REPO_ROOT = Path(__file__).absolute().parent.parent
+TORCHGEN_ROOT = Path(__file__).absolute().parent
+REPO_ROOT = TORCHGEN_ROOT.parent
 
 
 # Many of these functions share logic for defining both the definition
@@ -96,11 +97,13 @@ def context(msg_fn: Callable[[], str]) -> Iterator[None]:
         raise
 
 
-# A little trick from https://github.com/python/mypy/issues/6366
-# for getting mypy to do exhaustiveness checking
-# TODO: put this somewhere else, maybe
-def assert_never(x: NoReturn) -> NoReturn:
-    raise AssertionError(f"Unhandled type: {type(x).__name__}")
+if TYPE_CHECKING:
+    # A little trick from https://github.com/python/mypy/issues/6366
+    # for getting mypy to do exhaustiveness checking
+    # TODO: put this somewhere else, maybe
+    @deprecated("Use typing_extensions.assert_never instead")
+    def assert_never(x: NoReturn) -> NoReturn:  # type: ignore[misc] # noqa: F811
+        raise AssertionError(f"Unhandled type: {type(x).__name__}")
 
 
 @functools.cache
@@ -118,39 +121,47 @@ def string_stable_hash(s: str) -> int:
 # of what files have been written (so you can write out a list of output
 # files)
 class FileManager:
-    install_dir: str
-    template_dir: str
-    dry_run: bool
-    filenames: set[str]
-
-    def __init__(self, install_dir: str, template_dir: str, dry_run: bool) -> None:
-        self.install_dir = install_dir
-        self.template_dir = template_dir
-        self.filenames = set()
+    def __init__(
+        self,
+        install_dir: str | Path,
+        template_dir: str | Path,
+        dry_run: bool,
+    ) -> None:
+        self.install_dir = Path(install_dir)
+        self.template_dir = Path(template_dir)
+        self.files: set[Path] = set()
         self.dry_run = dry_run
 
-    def _write_if_changed(self, filename: str, contents: str) -> None:
-        old_contents: str | None
+    @property
+    def filenames(self) -> frozenset[str]:
+        return frozenset({file.as_posix() for file in self.files})
+
+    def _write_if_changed(self, filename: str | Path, contents: str) -> None:
+        file = Path(filename)
+        old_contents: str | None = None
         try:
-            with open(filename) as f:
-                old_contents = f.read()
+            old_contents = file.read_text(encoding="utf-8")
         except OSError:
-            old_contents = None
+            pass
         if contents != old_contents:
             # Create output directory if it doesn't exist
-            os.makedirs(os.path.dirname(filename), exist_ok=True)
-            with open(filename, "w") as f:
-                f.write(contents)
+            file.parent.mkdir(parents=True, exist_ok=True)
+            file.write_text(contents, encoding="utf-8")
 
     # Read from template file and replace pattern with callable (type could be dict or str).
     def substitute_with_template(
-        self, template_fn: str, env_callable: Callable[[], str | dict[str, Any]]
+        self,
+        template_fn: str | Path,
+        env_callable: Callable[[], str | dict[str, Any]],
     ) -> str:
-        template_path = os.path.join(self.template_dir, template_fn)
+        assert not Path(template_fn).is_absolute(), (
+            f"template_fn must be relative: {template_fn}"
+        )
+        template_path = self.template_dir / template_fn
         env = env_callable()
         if isinstance(env, dict):
             if "generated_comment" not in env:
-                generator_default = REPO_ROOT / "torchgen" / "gen.py"
+                generator_default = TORCHGEN_ROOT / "gen.py"
                 try:
                     generator = Path(
                         sys.modules["__main__"].__file__ or generator_default
@@ -170,38 +181,56 @@ class FileManager:
                     ),
                 }
             template = _read_template(template_path)
-            return template.substitute(env)
-        elif isinstance(env, str):
+            substitute_out = template.substitute(env)
+            # Ensure an extra blank line between the class/function definition
+            # and the docstring of the previous class/function definition.
+            # NB: It is generally not recommended to have docstrings in pyi stub
+            #     files. But if there are any, we need to ensure that the file
+            #     is properly formatted.
+            return re.sub(
+                r'''
+                (""")\n+             # match triple quotes
+                (
+                    (\s*@.+\n)*     # match decorators if any
+                    \s*(class|def)  # match class/function definition
+                )
+                ''',
+                r"\g<1>\n\n\g<2>",
+                substitute_out,
+                flags=re.VERBOSE,
+            )
+        if isinstance(env, str):
             return env
-        else:
-            assert_never(env)
+        assert_never(env)
 
     def write_with_template(
         self,
-        filename: str,
-        template_fn: str,
+        filename: str | Path,
+        template_fn: str | Path,
         env_callable: Callable[[], str | dict[str, Any]],
     ) -> None:
-        filename = f"{self.install_dir}/{filename}"
-        assert filename not in self.filenames, "duplicate file write {filename}"
-        self.filenames.add(filename)
+        filename = Path(filename)
+        assert not filename.is_absolute(), f"filename must be relative: {filename}"
+        file = self.install_dir / filename
+        assert file not in self.files, f"duplicate file write {file}"
+        self.files.add(file)
         if not self.dry_run:
             substitute_out = self.substitute_with_template(
                 template_fn=template_fn,
                 env_callable=env_callable,
             )
-            self._write_if_changed(filename=filename, contents=substitute_out)
+            self._write_if_changed(filename=file, contents=substitute_out)
 
     def write(
         self,
-        filename: str,
+        filename: str | Path,
         env_callable: Callable[[], str | dict[str, Any]],
     ) -> None:
         self.write_with_template(filename, filename, env_callable)
 
     def write_sharded(
         self,
-        filename: str,
+        filename: str | Path,
         items: Iterable[T],
         *,
         key_fn: Callable[[T], str],
@@ -223,8 +252,8 @@ class FileManager:
 
     def write_sharded_with_template(
         self,
-        filename: str,
-        template_fn: str,
+        filename: str | Path,
+        template_fn: str | Path,
         items: Iterable[T],
         *,
         key_fn: Callable[[T], str],
@@ -233,6 +262,8 @@ class FileManager:
         base_env: dict[str, Any] | None = None,
         sharded_keys: set[str],
     ) -> None:
+        file = Path(filename)
+        assert not file.is_absolute(), f"filename must be relative: {filename}"
         everything: dict[str, Any] = {"shard_id": "Everything"}
         shards: list[dict[str, Any]] = [
             {"shard_id": f"_{i}"} for i in range(num_shards)
@@ -270,31 +301,27 @@ class FileManager:
             merge_env(shards[sid], env)
             merge_env(everything, env)
 
-        dot_pos = filename.rfind(".")
-        if dot_pos == -1:
-            dot_pos = len(filename)
-        base_filename = filename[:dot_pos]
-        extension = filename[dot_pos:]
-
         for shard in all_shards:
             shard_id = shard["shard_id"]
             self.write_with_template(
-                f"{base_filename}{shard_id}{extension}",
+                file.with_stem(f"{file.stem}{shard_id}"),
                 template_fn,
                 lambda: shard,
             )
 
         # filenames is used to track compiled files, but FooEverything.cpp isn't meant to be compiled
-        self.filenames.discard(
-            f"{self.install_dir}/{base_filename}Everything{extension}"
-        )
+        self.files.discard(self.install_dir / file.with_stem(f"{file.stem}Everything"))
 
-    def write_outputs(self, variable_name: str, filename: str) -> None:
-        """Write a file containing the list of all outputs which are
-        generated by this script."""
-        content = "set({}\n    {})".format(
-            variable_name,
-            "\n    ".join('"' + name + '"' for name in sorted(self.filenames)),
+    def write_outputs(self, variable_name: str, filename: str | Path) -> None:
+        """Write a file containing the list of all outputs which are generated by this script."""
+        content = "\n".join(
+            (
+                "set(",
+                variable_name,
+                # Use POSIX paths to avoid invalid escape sequences on Windows
+                *(f'    "{file.as_posix()}"' for file in sorted(self.files)),
+                ")",
+            )
         )
         self._write_if_changed(filename, content)
 
@@ -309,12 +336,15 @@ class FileManager:
 
 # Helper function to generate file manager
 def make_file_manager(
-    options: Namespace, install_dir: str | None = None
+    options: Namespace,
+    install_dir: str | Path | None = None,
 ) -> FileManager:
     template_dir = os.path.join(options.source_path, "templates")
     install_dir = install_dir if install_dir else options.install_dir
     return FileManager(
-        install_dir=install_dir, template_dir=template_dir, dry_run=options.dry_run
+        install_dir=install_dir,
+        template_dir=template_dir,
+        dry_run=options.dry_run,
     )
 
 
@@ -437,7 +467,10 @@ class NamespaceHelper:
     """
 
     def __init__(
-        self, namespace_str: str, entity_name: str = "", max_level: int = 2
+        self,
+        namespace_str: str,
+        entity_name: str = "",
+        max_level: int = 2,
     ) -> None:
         # cpp_namespace can be a colon joined string such as torch::lazy
         cpp_namespaces = namespace_str.split("::")
@@ -454,7 +487,8 @@ class NamespaceHelper:
 
     @staticmethod
     def from_namespaced_entity(
-        namespaced_entity: str, max_level: int = 2
+        namespaced_entity: str,
+        max_level: int = 2,
     ) -> NamespaceHelper:
         """
         Generate helper from nested namespaces as long as class/function name. E.g.: "torch::lazy::add"