CrossValidator, TrainValidationSplit should preserve all models after fitting

I've heard multiple requests for having CrossValidatorModel and TrainValidationSplitModel preserve the full list of fitted models. This sounds very valuable.

One decision should be made before we do this: Should we save and load the models in ML persistence? That could blow up the size of a saved Pipeline if the models are large.

• I suggest not saving the models by default but allowing saving if specified. We could specify whether to save the model as an extra Param for CrossValidatorModelWriter, but we would have to make sure to expose CrossValidatorModelWriter as a public API and modify the return type of CrossValidatorModel.write to be CrossValidatorModelWriter (but this will not be a breaking change).