[Misc] Improve type annotations for `support_torch_compile` (#10763)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

vllm/compilation/decorators.py

 import inspect
-from typing import Dict, List, Optional, Union
+from typing import Callable, Dict, List, Optional, TypeVar, Union, overload
 
 import torch
+import torch.nn as nn
 
 from vllm.compilation.counter import compilation_counter
 from vllm.compilation.wrapper import TorchCompileWrapperWithCustomDispatcher
@@ -12,10 +13,27 @@ from vllm.utils import supports_dynamo
 
 logger = init_logger(__name__)
 
+_T = TypeVar("_T", bound=type[nn.Module])
+
+
+@overload
+def support_torch_compile(
+    *,
+    dynamic_arg_dims: Optional[Dict[str, Union[int, List[int]]]],
+) -> Callable[[_T], _T]:
+    ...
+
+
+@overload
+def support_torch_compile(cls: _T) -> _T:
+    ...
+
 
 def support_torch_compile(
-        cls: Optional[type] = None,
-        dynamic_arg_dims: Optional[Dict[str, Union[int, List[int]]]] = None):
+    cls: Optional[_T] = None,
+    *,
+    dynamic_arg_dims: Optional[Dict[str, Union[int, List[int]]]] = None,
+) -> Union[Callable[[_T], _T], _T]:
     """
     A decorator to add support for compiling the forward method of a class.
 
@@ -66,7 +84,7 @@ def support_torch_compile(
     computation graph.
     """
 
-    def cls_decorator_helper(cls: type):
+    def cls_decorator_helper(cls: _T) -> _T:
         # helper to pass `dynamic_arg_dims`` to `_support_torch_compile``
         # to avoid too much indentation for `_support_torch_compile``
         if not hasattr(cls, 'forward'):
@@ -105,8 +123,10 @@ def support_torch_compile(
     return cls_decorator_helper
 
 
-def _support_torch_compile(cls: type,
-                           dynamic_arg_dims: Dict[str, Union[int, List[int]]]):
+def _support_torch_compile(
+    cls: _T,
+    dynamic_arg_dims: Dict[str, Union[int, List[int]]],
+) -> _T:
     """
     A decorator to add support for compiling the forward method of a class.
     """
@@ -119,7 +139,7 @@ def _support_torch_compile(cls: type,
     #  other than TorchCompileWrapperWithCustomDispatcher
     cls.__bases__ = cls.__bases__ + (TorchCompileWrapperWithCustomDispatcher, )
 
-    old_init = cls.__init__  # type: ignore
+    old_init = cls.__init__
 
     def __init__(self, *, vllm_config: VllmConfig, prefix: str = '', **kwargs):
         old_init(self, vllm_config=vllm_config, prefix=prefix, **kwargs)
@@ -135,7 +155,7 @@ def _support_torch_compile(cls: type,
         TorchCompileWrapperWithCustomDispatcher.__init__(
             self, compilation_level=vllm_config.compilation_config.level)
 
-    cls.__init__ = __init__  # type: ignore
+    cls.__init__ = __init__
 
     def __call__(self, *args, **kwargs):
         # torch.compiler.is_compiling() means we are inside the compilation
@@ -180,5 +200,5 @@ def _support_torch_compile(cls: type,
             model_output = self.forward(*args, **kwargs)
             return model_output
 
-    cls.__call__ = __call__  # type: ignore
+    cls.__call__ = __call__
     return cls