Rework automatic code samples in docstrings
# What does this PR do?

This PR reworks the automatic code sample docstrings in two ways:

First, use the auto-classes for the preprocessing. As was decided internally, we want to document the model class used, but use the auto classes for preprocessing so users are not confused when a given model uses the tokenizer/feature extractor/image processor/processor of another.

Second, we don't want to showcase `hf-internal-testing` models in the docstrings. Those are tiny random models and it confuses users more than it helps. However when using the standard checkpoint we get doctest problems, so this PR removes the output/loss from the code example when it shouldn't be tested.

Two examples are shown with BERT and DeBERTaV2, I can add more models to the PR if it suits everyone.
_The documentation is not available anymore as the PR was closed or merged._