Dummy labels no longer on by default in `to_tf_dataset`
@lhoestq Let me make sure we never need it, and if not then I'll remove it entirely in a follow-up PR. Thanks ;) it will be less confusing and easier to maintain to not keep unused hacky features
After more experimentation, I think I have a way to do things that doesn't depend on adding `dummy_labels` - they were quite a hacky solution anyway!