grad_mode decorators without paren (#107086)
This PR implements the feature described in #107036 for `no_grad`, `enable_grad` and `inference_mode`.
Users can still use the above as before but they can also use them without parentheses.
For example:
```python
import torch
a = torch.ones(1, requires_grad=True)
def do_something():
print(2 * a)
with torch.no_grad():
do_something() # tensor([2.])
torch.no_grad()(do_something)() # tensor([2.])
torch.no_grad(do_something)() # tensor([2.])
do_something() # tensor([2.], grad_fn=<MulBackward0>)
```
For `inference_mode`, decorating without parenthesis is equivalent to decorating with the default `mode=True`, similiar to how dataclasses behave (https://docs.python.org/3/library/dataclasses.html#module-contents)
Closes #107036
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107086
Approved by: https://github.com/albanD
