[AMP] Add deprecated decorator for torch.xxx.amp.autocast class (#163654)

As the title stated.

**Changes:**
- torch.cuda.amp.autocast
- torch.cpu.amp.autocast
- add explicit `__new__` and `__init_subclass__` for those class above for inspect.signature to retrieve correct signature

Pull Request resolved: https://github.com/pytorch/pytorch/pull/163654
Approved by: https://github.com/Skylion007

torch/cpu/amp/autocast_mode.py

@@ -1,4 +1,5 @@
 # mypy: allow-untyped-defs
+import sys
 from typing import Any
 from typing_extensions import deprecated
 
@@ -8,17 +9,36 @@ import torch
 __all__ = ["autocast"]
 
 
+@deprecated(
+    "`torch.cpu.amp.autocast(args...)` is deprecated. "
+    "Please use `torch.amp.autocast('cpu', args...)` instead.",
+    category=FutureWarning,
+)
 class autocast(torch.amp.autocast_mode.autocast):
     r"""
     See :class:`torch.autocast`.
     ``torch.cpu.amp.autocast(args...)`` is deprecated. Please use ``torch.amp.autocast("cpu", args...)`` instead.
     """
 
-    @deprecated(
-        "`torch.cpu.amp.autocast(args...)` is deprecated. "
-        "Please use `torch.amp.autocast('cpu', args...)` instead.",
-        category=FutureWarning,
-    )
+    # TODO: remove this conditional once we stop supporting Python < 3.13
+    # Prior to Python 3.13, inspect.signature could not retrieve the correct
+    # signature information for classes decorated with @deprecated (unless
+    # the __new__ static method was explicitly defined);
+    #
+    # However, this issue has been fixed in Python 3.13 and later versions.
+    if sys.version_info < (3, 13):
+
+        def __new__(
+            cls,
+            enabled: bool = True,
+            dtype: torch.dtype = torch.bfloat16,
+            cache_enabled: bool = True,
+        ):
+            return super().__new__(cls)
+
+        def __init_subclass__(cls):
+            pass
+
     def __init__(
         self,
         enabled: bool = True,

torch/cuda/amp/autocast_mode.py

@@ -1,5 +1,6 @@
 # mypy: allow-untyped-defs
 import functools
+import sys
 from typing import Any
 from typing_extensions import deprecated
 
@@ -9,17 +10,36 @@ import torch
 __all__ = ["autocast", "custom_fwd", "custom_bwd"]
 
 
+@deprecated(
+    "`torch.cuda.amp.autocast(args...)` is deprecated. "
+    "Please use `torch.amp.autocast('cuda', args...)` instead.",
+    category=FutureWarning,
+)
 class autocast(torch.amp.autocast_mode.autocast):
     r"""See :class:`torch.autocast`.
 
     ``torch.cuda.amp.autocast(args...)`` is deprecated. Please use ``torch.amp.autocast("cuda", args...)`` instead.
     """
 
-    @deprecated(
-        "`torch.cuda.amp.autocast(args...)` is deprecated. "
-        "Please use `torch.amp.autocast('cuda', args...)` instead.",
-        category=FutureWarning,
-    )
+    # TODO: remove this conditional once we stop supporting Python < 3.13
+    # Prior to Python 3.13, inspect.signature could not retrieve the correct
+    # signature information for classes decorated with @deprecated (unless
+    # the __new__ static method was explicitly defined);
+    #
+    # However, this issue has been fixed in Python 3.13 and later versions.
+    if sys.version_info < (3, 13):
+
+        def __new__(
+            cls,
+            enabled: bool = True,
+            dtype: torch.dtype = torch.float16,
+            cache_enabled: bool = True,
+        ):
+            return super().__new__(cls)
+
+        def __init_subclass__(cls):
+            pass
+
     def __init__(
         self,
         enabled: bool = True,