Fix to_tf_dataset references in docs
The code snippet in [this section](https://huggingface.co/docs/datasets/master/use_dataset.html?highlight=to_tf_dataset#tensorflow) is missing an import (`DataCollatorWithPadding`) and doesn't initialize the TF model before the `model.fit` call.
Fix the `to_tf_dataset` references in the docs. The currently failing example of usage will be fixed by #3338.  