Add support for data transformations

[scnakandala]

Currrently, we assume the entire dataset will be used to train the model.
But this is not the case. Only some columns may be selected by a particular model, or even some rows (e.g., grouped learning) or both.

Data selection logic should be a part of the hyperparameter search space and estimators should take care of it.

Have to handle data partitions being of different sizes.