TF: Update multiple choice example
# What does this PR do?

Updates the TF example for multiple choice.

This example update is not as clean as the previous ones -- despite the deletion of a custom dataset function, it required the addition of a custom data collator ([just like in its PT counterpart](https://github.com/huggingface/transformers/blob/master/examples/pytorch/multiple-choice/run_swag.py#L150)).
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15868). All of your documentation changes will be reflected on that endpoint.