Fix method stub used for fixing mypy issue to work with pylint (#42356)
Summary:
Make function from method
Since _forward_unimplemented is defined within the nn.Module class,
pylint (correctly) complains about not implementing this method in subclasses.
Fixes https://github.com/pytorch/pytorch/issues/42305
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42356
Reviewed By: mruberry
Differential Revision: D22867255
Pulled By: ezyang
fbshipit-source-id: ccf3e45e359d927e010791fadf70b2ef231ddb0b
