Support @torch.jit.unused on a @torch.no_grad decorated function (#41496)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41496
use the wrapped function (instead of the wrapper) to obtain argument names
Test Plan:
```
buck test mode/dev-nosan //caffe2/test:jit -- 'test_unused_decorator \(test_jit\.TestScript\)'
```
Before:
```
> Traceback (most recent call last):
> File "/data/users/yuxinwu/fbsource2/fbcode/buck-out/dev/gen/caffe2/test/jit#binary,link-tree/test_jit.py", line 3014, in test_unused_decorator
> torch.jit.script(MyMod())
> File "/data/users/yuxinwu/fbsource2/fbcode/buck-out/dev/gen/caffe2/test/jit#binary,link-tree/torch/jit/_script.py", line 888, in script
> obj, torch.jit._recursive.infer_methods_to_compile
> File "/data/users/yuxinwu/fbsource2/fbcode/buck-out/dev/gen/caffe2/test/jit#binary,link-tree/torch/jit/_recursive.py", line 317, in create_script_module
> return create_script_module_impl(nn_module, concrete_type, stubs_fn)
> File "/data/users/yuxinwu/fbsource2/fbcode/buck-out/dev/gen/caffe2/test/jit#binary,link-tree/torch/jit/_recursive.py", line 376, in create_script_module_impl
> create_methods_from_stubs(concrete_type, stubs)
> File "/data/users/yuxinwu/fbsource2/fbcode/buck-out/dev/gen/caffe2/test/jit#binary,link-tree/torch/jit/_recursive.py", line 292, in create_methods_from_stubs
> concrete_type._create_methods(defs, rcbs, defaults)
> RuntimeError:
> Non-static method does not have a self argument:
> File "/data/users/yuxinwu/fbsource2/fbcode/buck-out/dev/gen/caffe2/test/jit#binary,link-tree/test_jit.py", line 3012
> def forward(self, x):
> return self.fn(x)
> ~~~~~~~ <--- HERE
>
```
Reviewed By: eellison
Differential Revision: D22554479
fbshipit-source-id: 03e432ea92ed973cc57ff044da80ae7a36f6af4c
