Generate: skip left-padding tests on old models
# What does this PR do?

We have a few left-padding tests polluting our daily CI -- these are from old models, where it is not worth committing >1hr per model to add support for left padding.

This PR skips the test in those models, so we can focus our energy where it matters :)
_The documentation is not available anymore as the PR was closed or merged._<|||||>It seems like `GPTBigCode` need to be fixed! Adding back the `@slow` to defer back fixing<|||||>@gante it all passes now 😄 
