This PR adds .iter_torch_batches() and .iter_tf_batches() convenience APIs, which takes care of ML framework tensor conversion, the narrow tensor waste for the .iter_batches() call ("numpy" format), and unifies batch formats around two options: a single tensor for simple/pure-tensor/single-column datasets, and a dictionary of tensors for multi-column datasets.
The package "ml" should be renamed to "air".
Main question: Keep a `ml.py` with `from ray.air import *` for some level of backwards compatibility?
I'd go for no to force people to use the new structure.
