Add disclaimer for necessary fake models
# What does this PR do?

This PR adds a disclaimer for the fake models we can't really remove since the canonical checkpoint is very big already. You can see the result in the [preview](https://moon-ci-docs.huggingface.co/docs/transformers/pr_21178/en/model_doc/gptj#transformers.GPTJModel.forward.example).

If this is acceptable, I'll add it to the other places where we want a tiny random model instead of the huge one for the docstrings.
_The documentation is not available anymore as the PR was closed or merged._