mypy is questionable

It seems that mypy has not discovered wrong return type typehint in [scaaml.io.Dataset.as_tfdataset](https://github.com/google/scaaml/blob/8b7e5ff700005c393f48aa1b22530f0f5a784bc4/scaaml/io/dataset.py#L380).

There are two issues:

1. Fixing the typehint.
2. Debugging and fixing mypy typechecking.

Python interpreter types the output as a tuple, when unpacking the types are tf.data.PrefetchDataset, dict, dict. The proper type should be Tuple[tf.data.Dataset, Dict, Dict]. Definitely not a Union of those.