to_tf_dataset() refactor
This is the promised cleanup to `to_tf_dataset()` now that the course is out of the way! The main changes are:

- A collator is always required (there was way too much hackiness making things like labels work without it)
- Lots of cleanup and a lot of code moved to `_get_output_signature`
- Should now handle it gracefully when the data collator adds unexpected columns