Add _CHECKPOINT_FOR_DOC to all models
This adds the `_CHECKPOINT_FOR_DOC` variable for all models that do not have it. This value is used in the pipeline tests refactoring, where all models/tokenizers compatible with a given pipeline are tested. To that end, it is important to have a source of truth as to which checkpoint is well maintained, which should be the case of the `_CHECKPOINT_FOR_DOC` that is used in the documentation.