[BE] wrap deprecated function/class with typing_extensions.deprecated (#127689)

Use `typing_extensions.deprecated` for deprecation annotation if possible. Otherwise, add `category=FutureWarning` to `warnings.warn("message")` if the category is missing.

Note that only warnings that their messages contain `[Dd]eprecat(ed|ion)` are updated in this PR.

Resolves #126888

- #126888

This PR is split from PR #126898.

- #126898

------

Pull Request resolved: https://github.com/pytorch/pytorch/pull/127689
Approved by: https://github.com/Skylion007

torch/utils/_cxx_pytree.py

@@ -15,7 +15,6 @@ collection support for PyTorch APIs.
 import functools
 import sys
 import types
-import warnings
 from typing import (
     Any,
     Callable,
@@ -28,6 +27,7 @@ from typing import (
     TypeVar,
     Union,
 )
+from typing_extensions import deprecated
 
 import torch
 
@@ -167,6 +167,11 @@ def register_pytree_node(
     )
 
 
+@deprecated(
+    "`torch.utils._cxx_pytree._register_pytree_node` is deprecated. "
+    "Please use `torch.utils._cxx_pytree.register_pytree_node` instead.",
+    category=FutureWarning,
+)
 def _register_pytree_node(
     cls: Type[Any],
     flatten_fn: FlattenFunc,
@@ -207,11 +212,6 @@ def _register_pytree_node(
             original context. This is used for json deserialization, which is being used in
             :mod:`torch.export` right now.
     """
-    warnings.warn(
-        "torch.utils._cxx_pytree._register_pytree_node is deprecated. "
-        "Please use torch.utils._cxx_pytree.register_pytree_node instead.",
-        stacklevel=2,
-    )
 
     _private_register_pytree_node(
         cls,