TF: Update QA example
# What does this PR do?

Another example update, this time for question answering. Removes the dummy loss (we can use the internal loss) and the custom dataset preparation.

@Rocketknight1 in this example, the evaluation dataset has some custom handling. Do you think it is worth diving into?
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15870). All of your documentation changes will be reflected on that endpoint.<|||||>@gante If you mean the bit that's in the PT example too, then we probably have to keep that! (And you don't need to change it for the refactor)