Deprecate decorating classes with torch.no_grad and similar (#89522)
Fixes https://github.com/pytorch/pytorch/issues/89450
I would have completely removed it but I don't think this is particularly urgent and there are some use of it in the wild: https://github.com/search?q=%2Ftorch%5C.no_grad%5C%28%5C%29%5Cnclass%2F&type=code
So we might as well take one release to do it.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89522
Approved by: https://github.com/lezcano, https://github.com/soulitzer, https://github.com/janeyx99
