Modify API for advanced settings (GAM learners)

[abgoswam]

Towards #1798 .

This PR addresses the following algos

• BinaryClassificationGamTrainer
• RegressionGamTrainer

The following changes have been made:

1. Two public extension methods, one for simple arguments and the other for advanced options
2. Make constructors internal
3. Pass Options objects as arguments instead of Action delegate
4. Rename Arguments to Options
5. Rename Options objects as options (instead of args or advancedSettings used so far)

[sfilipi]

_options = options; [](start = 10, length = 21)

check #Resolved

[sfilipi]

[rogancarr]

🚢 🇮🇹

[rogancarr]

One thing that I've noticed about this switch to options is that we have two APIs for each learner: One with options, and one with a subset of the options spelled out. Wouldn't it make sense to only have one constructor that took options, and have the Catalog interface handle the multiplicity of ways to construct it?

That is, only have

new MyTrainer(env, options)

And then have the catalog do this:

public static BinaryClassificationGamTrainer GeneralizedAdditiveModels(this BinaryClassificationCatalog.BinaryClassificationTrainers catalog,
            string labelColumn = DefaultColumnNames.Label,
            string featureColumn = DefaultColumnNames.Features,
            string weights = null,
            int numIterations = GamDefaults.NumIterations,
            double learningRate = GamDefaults.LearningRates,
            int maxBins = GamDefaults.MaxBins)
        {
            Contracts.CheckValue(catalog, nameof(catalog));
            var env = CatalogUtils.GetEnvironment(catalog);
            var options = new BinaryClassificationGamTrainer.Options{
                xyz = etc
            }
            return new BinaryClassificationGamTrainer(env, options);
        }
``` #Resolved

[abgoswam]

Indeed. That's issue #2100

Recall, we had issues with the constructor unification for FastTreeRanking.

P0 is to get the public APIs ready.
#2100 is P1/P2 in that context.

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In reply to: 249970882 [](ancestors = 249970882)