This adds a per-dataset config object to DataParallelTrainer. These configs define how the Dataset should be read into the DataParallelTrainer. It configures the preprocessing, splitting, and ingest strategy per-dataset. DataParallelTrainers declare default DatasetConfigs for each dataset passed in the ``datasets`` argument. Users have the opportunity to selectively override these configs by passing the ``dataset_config`` argument. Trainers can also define user customizable values (e.g., XGBoostTrainer doesn't support streaming ingest).
This PR adds the minimal support for dataset configs. Future PRs will:
- Add support for streaming ingest
- Move this config from DataParallelTrainer to ml.Trainer
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.
