Check diff files in check_copies

[zucchini-nlp]

Feature request

As mentioned in this comment we want to check that files generated by diff are not changed whenever we run make fix-copies. That way we can catch if there are any any changes in super classes that aren't being propagated to the diff child.

Motivation

Repo consistency checks

Your contribution

Tagging @ArthurZucker for diff and maybe @ydshieh for repo consistency check. I will be very slow if I start adding this