The curious case of TrainedWrapperEstimatorBase and friends

[TomFinley]

So, while I was doing another round of internalization, one thing that I internalized was this. (Basically, something to handle the shimming from the now internal IDataTransform interface to the new ITransformer interface, during the regrettable situations -- thankfully few -- where such a thing is still necessary. So this:

machinelearning/src/Microsoft.ML.Data/DataLoadSave/TransformWrapper.cs

Line 20 in 63a7654

public sealed class TransformWrapper : ITransformer

and this

machinelearning/src/Microsoft.ML.Data/DataLoadSave/TransformWrapper.cs

Line 151 in 63a7654

public abstract class TrainedWrapperEstimatorBase : IEstimator<TransformWrapper>

Now, that's all fine, but after doing the necessary work it seemed that I could delete the estimator wrapper entirely, but then I see this very intriguing note.

machinelearning/src/Microsoft.ML.Data/DataView/RowToRowMapperTransform.cs

Lines 50 to 56 in 63a7654

	/// <summary>
	/// DO NOT USE IT!
	/// Purpose of this method is to enable legacy loading and unwrapping of RowToRowTransform.
	/// It should be removed as soon as we get rid of <see cref="TrainedWrapperEstimatorBase"/>
	/// Returns parent transfomer which uses this mapper.
	/// </summary>
	ITransformer GetTransformer();

I do not understand what is going on here. This is not essential -- everything here is internal -- but it seems at least odd. This seems to indicate that this method and the class I wanted to delete have something to do with each other, but as far as I can tell they have nothing to do with each other whatsoever. But then why the comment?

Anyway, there's clearly something odd going on. This is absolutely not critical, but I wanted to register an issue about the oddness, since the code underlying it passes my understanding.