FastTree EarlyStoppingRule definition is inconsistent with API

[rogancarr]

In FastTree, the EarlyStoppingRule is defined as

[Argument(ArgumentType.Multiple, HelpText = "Early stopping rule. (Validation set (/valid) is required.)", ShortName = "esr", NullName = "<Disable>")]
[TGUI(Label = "Early Stopping Rule", Description = "Early stopping rule. (Validation set (/valid) is required.)")]
public IEarlyStoppingCriterionFactory EarlyStoppingRule;

This can be specified like so:

var fastTreeTrainer = mlContext.Regression.Trainers.FastTree(new 
    Trainers.FastTree.FastTreeRegressionTrainer.Options {
        EarlyStoppingMetrics = 2,
        EarlyStoppingRule = new GLEarlyStoppingCriterion.Arguments()
    });

This exposes the IComponentFactory way of doing things in the public API, which seems to be inconsistent with the public API. One suggestion is to use an enum over the existing options.

That said, this does support custom early stopping methods through implementing IEarlyStoppingCriterionFactory.

[wschin]

It will be a part of PR #2753 which will fix #2617.

[wschin]

We can't make them enum because they all have their own options such as WindowSize below.

    public sealed class UPEarlyStoppingCriterion : EarlyStoppingCriterion<UPEarlyStoppingCriterion.Options>
    {
        public sealed class Options : OptionsBase, IEarlyStoppingCriterionFactory
        {
            public int WindowSize = 5;
        }