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.
Adds a Dataset.split_proportionately method that allows the user to split a dataset using proportions. This is a very common use-case for eg. train-test splitting. The implementation is a thin wrapper over Dataset.split_at_indices.
Additionally, this PR adds a ray.ml.train_test_split function intended to provide a familiar API to ML practitioners.
