Clean views created inside no_grad that are modified inplace (#32839)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32839
As mentioned in the updated comment in `variable.h`, this disambiguate code like:
```python
base = torch.rand(10, requires_grad=True)
with torch.no_grad():
view = base[1]
view.copy_(var)
torch.autograd.grad(base.sum(), var) # <- what should it return?
```
Given that there is no consensus of what should happen here (does the gradient flow through the view in the no_grad or not). This special case is detected and forbidden.
As mentionned in the error message:
- If you want it to be tracked: move both out of the no_grad
- If do not want them to be tracked, move both inside the no_grad
This implies that any custom Function that returns views does not allow inplace modification on its output. I'll add a PR to the stack to relax this to be a DeprecationWarning for now. And we will make it into an actual error for 1.6
This replaces https://github.com/pytorch/pytorch/pull/26607
cc sublee
Test Plan: Imported from OSS
Differential Revision: D19814114
Pulled By: albanD
fbshipit-source-id: ff2c9d97c8f876d9c31773a2170e37b06d88bed7
