Refactor GAM Predictor to be Public-ly Creatable (#2142)

Refactoring the GAM Trainer / Predictor to move all Training information into the trainer, and make the GAM predictor public so that models can be instantiated from generic parameters.

src/Microsoft.ML.FastTree/GamClassification.cs

@@ -108,8 +108,8 @@ private static bool[] ConvertTargetsToBool(double[] targets)
         private protected override IPredictorProducing<float> TrainModelCore(TrainContext context)
         {
             TrainBase(context);
-            var predictor = new BinaryClassificationGamModelParameters(Host, InputLength, TrainSet,
-                MeanEffect, BinEffects, FeatureMap);
+            var predictor = new BinaryClassificationGamModelParameters(Host,
+                BinUpperBounds, BinEffects, MeanEffect, InputLength, FeatureMap);
             var calibrator = new PlattCalibrator(Host, -1.0 * _sigmoidParameter, 0);
             return new CalibratedPredictor(Host, predictor, calibrator);
         }
@@ -157,14 +157,29 @@ protected override SchemaShape.Column[] GetOutputColumnsCore(SchemaShape inputSc
         }
     }
 
-    public class BinaryClassificationGamModelParameters : GamModelParametersBase, IPredictorProducing<float>
+    /// <summary>
+    /// The model parameters class for Binary Classification GAMs
+    /// </summary>
+    public sealed class BinaryClassificationGamModelParameters : GamModelParametersBase, IPredictorProducing<float>
     {
         internal const string LoaderSignature = "BinaryClassGamPredictor";
         public override PredictionKind PredictionKind => PredictionKind.BinaryClassification;
 
-        internal BinaryClassificationGamModelParameters(IHostEnvironment env, int inputLength, Dataset trainset,
-            double meanEffect, double[][] binEffects, int[] featureMap)
-            : base(env, LoaderSignature, inputLength, trainset, meanEffect, binEffects, featureMap) { }
+        /// <summary>
+        /// Construct a new Binary Classification GAM with the defined properties.
+        /// </summary>
+        /// <param name="env">The Host Environment</param>
+        /// <param name="binUpperBounds">An array of arrays of bin-upper-bounds for each feature.</param>
+        /// <param name="binEffects">Anay array of arrays of effect sizes for each bin for each feature.</param>
+        /// <param name="intercept">The intercept term for the model. Also referred to as the bias or the mean effect.</param>
+        /// <param name="inputLength">The number of features passed from the dataset. Used when the number of input features is
+        /// different than the number of shape functions. Use default if all features have a shape function.</param>
+        /// <param name="featureToInputMap">A map from the feature shape functions (as described by the binUpperBounds and BinEffects)
+        /// to the input feature. Used when the number of input features is different than the number of shape functions. Use default if all features have
+        /// a shape function.</param>
+        public BinaryClassificationGamModelParameters(IHostEnvironment env,
+            double[][] binUpperBounds, double[][] binEffects, double intercept, int inputLength, int[] featureToInputMap)
+            : base(env, LoaderSignature, binUpperBounds, binEffects, intercept, inputLength, featureToInputMap) { }
 
         private BinaryClassificationGamModelParameters(IHostEnvironment env, ModelLoadContext ctx)
             : base(env, LoaderSignature, ctx) { }
@@ -173,8 +188,10 @@ private static VersionInfo GetVersionInfo()
         {
             return new VersionInfo(
                 modelSignature: "GAM BINP",
-                verWrittenCur: 0x00010001,
-                verReadableCur: 0x00010001,
+                // verWrittenCur: 0x00010001, // Initial
+                // verWrittenCur: 0x00010001, // Added Intercept but collided from release 0.6-0.9
+                verWrittenCur: 0x00010002,    // Added Intercept (version revved to address collisions)
+                verReadableCur: 0x00010002,
                 verWeCanReadBack: 0x00010001,
                 loaderSignature: LoaderSignature,
                 loaderAssemblyName: typeof(BinaryClassificationGamModelParameters).Assembly.FullName);