Add all XxxPreTrainedModel to the main init
# What does this PR do?

This PR adds all XxxPreTrainedModel classes to the main init, making them public, and more generally, adds a CI quality check to make sure every object in the modeling files that is a subclass of `PreTrainedModel`, `TFPreTrainedModel` or `FlaxPreTrainedModel` is public (except the Encoder, Decoder, Wrapper and the ones explicitly listed as privates).

Fixes #12193