Fix inference_mode decorator (#68617)
Summary:
This fixes the case when `torch.inference_mode` is called with `mode=False` (disabled). When used as a decorator, it ignored the argument and enabled inference mode anyway.
`_DecoratorContextManager` is changed so that a new instance is a copy instead of a new instance with default parameters.
I also added more tests to cover this case.
Current behaviour:
```python
>>> import torch
>>> x = torch.ones(1, 2, 3, requires_grad=True)
>>> torch.inference_mode(mode=False)
... def func(x):
... return x * x
...
>>> out = func(x)
>>> out.requires_grad
False
```
New behaviour (fixed):
```python
>>> import torch
>>> x = torch.ones(1, 2, 3, requires_grad=True)
>>> torch.inference_mode(mode=False)
... def func(x):
... return x * x
...
>>> out = func(x)
>>> out.requires_grad
True
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68617
Reviewed By: mrshenli
Differential Revision: D32958434
Pulled By: albanD
fbshipit-source-id: 133c69970ef8bffb9fc9ab5142dedcffc4c32945
