Fix weak module cuda() _flat_weights bug (#21107)
Summary:
Dynamically creating a type at runtime was messing up the MRO and has been causing many other problems. I think it's best to delete it, this causes a regression since
```python
self.linear = nn.Linear(10, 10)
isinstance(self.linear, nn.Linear)
```
will now be `False` again, but this will be fixed once recursive script mode is the default (#20939)
](https://our.intern.facebook.com/intern/diff/15560549/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21107
Pulled By: driazati
Differential Revision: D15560549
fbshipit-source-id: 7bd6b958acb4f353d427d66196bb4ee577ecb1a6
