fix: flake8-bugbear code B024 (#107265)
See #106571 item B024
This fix concerns the addition of `abstractmethod` to methods declared inside abstract classes.
Should I also include PEP8 compliant reformatting on the files I had to modify ?
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107265
Approved by: https://github.com/kit1980
