Remove custom function in no_grad block error message (#33896)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33896
Fixes #32625. Previously, we'd receive an error message if we have a
custom function return a view of an input in a no_grad block:
```
class Alias(Function):
staticmethod
def forward(ctx, x):
return x[:]
staticmethod
def backward(ctx, gx):
return gx
inp = torch.rand(2, requires_grad=True)
with torch.no_grad():
# Used to error out
output = Alias.apply(inp)
```
After this change, the error no longer happens. The behavior changes to
become consistent to if we had implemented an operator that does the
same thing as the custom function:
- the output requires_grad
- we are able to detect (and error out) if the user tries to modify the
output in-place outside of the no_grad block.
Test Plan: - new test
Differential Revision: D20345601
Pulled By: zou3519
fbshipit-source-id: 7f95b4254f52ddbf989d26f449660403bcde1c78
