Update TF LM examples
# What does this PR do?

As part of the objective to bring TF examples up to speed, this PR updates the LM examples with the internal loss computations and the `to_tf_dataset()` functionality, which shaves off significant custom code.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15855). All of your documentation changes will be reflected on that endpoint.