Make errors for loss-less models more user-friendly
# What does this PR do?

A common mistake beginners encounter is trying to fine-tune with the `Trainer` one of the AutoModel which do not have any head, and can't be fine-tuned directly. This PR makes the Trainer error at init when it received one such model, and also adds a more helpful error message when the outputs of the model don't have a loss.
_The documentation is not available anymore as the PR was closed or merged._