Fix syntax for class references
# What does this PR do?

This PR fixes the documentation links made of the form :obj:`~transformers.ObjectName`. Those are not detected by `doc-builder` (and can't really be added since we use `:obj:` very often) so I searched and fixed the docstrings instead.

See the [TextClassificationPipeline](https://huggingface.co/docs/transformers/master/en/main_classes/pipelines#transformers.TextClassificationPipeline) in the current doc for an example of the problem:

![image](https://user-images.githubusercontent.com/35901082/144895213-013c0bce-5f40-499a-8631-bd97adb3720e.png)

cc @mishig25 @LysandreJik (merging to fix but will address any comment you have on your return Lysandre).
LGTM, thanks for the ping