Update calibrator estimators to be more suitable.

* Internalize infrastructure only interface ICalibratorTrainer.
* Update calibrator estimators so they are a suitable replacement
  for calibrator trainers in the public surface, e.g., no longer
  take IPredictor.

Author: TomFinley

docs/samples/Microsoft.ML.Samples/Dynamic/Calibrator.cs

@@ -1,4 +1,8 @@
-using System;
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
 using System.Linq;
 using Microsoft.ML.Calibrator;
 using Microsoft.ML.Data;
@@ -75,7 +79,7 @@ public static void Calibration()
 
             // Let's train a calibrator estimator on this scored dataset. The trained calibrator estimator produces a transformer
             // that can transform the scored data by adding a new column names "Probability". 
-            var calibratorEstimator = new PlattCalibratorEstimator(mlContext, model, "Sentiment", "Features");
+            var calibratorEstimator = new PlattCalibratorEstimator(mlContext, "Sentiment", "Score");
             var calibratorTransformer = calibratorEstimator.Fit(scoredData);
 
             // Transform the scored data with a calibrator transfomer by adding a new column names "Probability". 

src/Microsoft.ML.Data/Prediction/Calibrator.cs

@@ -12,6 +12,7 @@
 using Microsoft.ML;
 using Microsoft.ML.Calibrator;
 using Microsoft.ML.CommandLine;
+using Microsoft.ML.Core.Data;
 using Microsoft.ML.Data;
 using Microsoft.ML.EntryPoints;
 using Microsoft.ML.Internal.Calibration;
@@ -85,14 +86,24 @@ namespace Microsoft.ML.Internal.Calibration
     /// <summary>
     /// Signature for the loaders of calibrators.
     /// </summary>
-    public delegate void SignatureCalibrator();
+    [BestFriend]
+    internal delegate void SignatureCalibrator();
 
+    [BestFriend]
     [TlcModule.ComponentKind("CalibratorTrainer")]
-    public interface ICalibratorTrainerFactory : IComponentFactory<ICalibratorTrainer>
+    internal interface ICalibratorTrainerFactory : IComponentFactory<ICalibratorTrainer>
     {
     }
 
-    public interface ICalibratorTrainer
+    /// <summary>
+    /// This is a legacy interface still used for the command line and entry-points. All applications should transition away
+    /// from this interface and still work instead via <see cref="IEstimator{TTransformer}"/> of <see cref="CalibratorTransformer{TICalibrator}"/>,
+    /// for example, the subclasses of <see cref="CalibratorEstimatorBase{TICalibrator}"/>. However for now we retain this
+    /// until such time as those components making use of it can transition to the new way. No public surface should use
+    /// this, and even new internal code should avoid its use if possible.
+    /// </summary>
+    [BestFriend]
+    internal interface ICalibratorTrainer
     {
         /// <summary>
         /// True if the calibrator needs training, false otherwise.
@@ -107,6 +118,17 @@ public interface ICalibratorTrainer
         ICalibrator FinishTraining(IChannel ch);
     }
 
+    /// <summary>
+    /// This is a shim interface implemented only by <see cref="CalibratorEstimatorBase{TICalibrator}"/> to enable
+    /// access to the underlying legacy <see cref="ICalibratorTrainer"/> interface for those components that use
+    /// that old mechanism that we do not care to change right now.
+    /// </summary>
+    [BestFriend]
+    internal interface IHaveCalibratorTrainer
+    {
+        ICalibratorTrainer CalibratorTrainer { get; }
+    }
+
     /// <summary>
     /// An interface for predictors that take care of their own calibration given an input data view.
     /// </summary>
@@ -842,6 +864,64 @@ public static IPredictor GetCalibratedPredictor(IHostEnvironment env, IChannel c
             return CreateCalibratedPredictor(env, (IPredictorProducing<float>)predictor, trainedCalibrator);
         }
 
+        public static ICalibrator TrainCalibrator(IHostEnvironment env, IChannel ch, ICalibratorTrainer caliTrainer, IDataView scored, string labelColumn, string scoreColumn, string weightColumn = null, int maxRows = _maxCalibrationExamples)
+        {
+            Contracts.CheckValue(env, nameof(env));
+            env.CheckValue(ch, nameof(ch));
+            ch.CheckValue(scored, nameof(scored));
+            ch.CheckValue(caliTrainer, nameof(caliTrainer));
+            ch.CheckParam(!caliTrainer.NeedsTraining || !string.IsNullOrWhiteSpace(labelColumn), nameof(labelColumn),
+                "If " + nameof(caliTrainer) + " requires training, then " + nameof(labelColumn) + " must have a value.");
+            ch.CheckNonWhiteSpace(scoreColumn, nameof(scoreColumn));
+
+            if (!caliTrainer.NeedsTraining)
+                return caliTrainer.FinishTraining(ch);
+
+            var labelCol = scored.Schema[labelColumn];
+            var scoreCol = scored.Schema[scoreColumn];
+
+            var weightCol = weightColumn == null ? null : scored.Schema.GetColumnOrNull(weightColumn);
+            if (weightColumn != null && !weightCol.HasValue)
+                throw ch.ExceptSchemaMismatch(nameof(weightColumn), "weight", weightColumn);
+
+            ch.Info("Training calibrator.");
+
+            var cols = weightCol.HasValue ?
+                new Schema.Column[] { labelCol, scoreCol, weightCol.Value } :
+                new Schema.Column[] { labelCol, scoreCol };
+
+            using (var cursor = scored.GetRowCursor(cols))
+            {
+                var labelGetter = RowCursorUtils.GetLabelGetter(cursor, labelCol.Index);
+                var scoreGetter = RowCursorUtils.GetGetterAs<Single>(NumberType.R4, cursor, scoreCol.Index);
+                ValueGetter<Single> weightGetter = !weightCol.HasValue ? (ref float dst) => dst = 1 :
+                    RowCursorUtils.GetGetterAs<Single>(NumberType.R4, cursor, weightCol.Value.Index);
+
+                int num = 0;
+                while (cursor.MoveNext())
+                {
+                    Single label = 0;
+                    labelGetter(ref label);
+                    if (!FloatUtils.IsFinite(label))
+                        continue;
+                    Single score = 0;
+                    scoreGetter(ref score);
+                    if (!FloatUtils.IsFinite(score))
+                        continue;
+                    Single weight = 0;
+                    weightGetter(ref weight);
+                    if (!FloatUtils.IsFinite(weight))
+                        continue;
+
+                    caliTrainer.ProcessTrainingExample(score, label > 0, weight);
+
+                    if (maxRows > 0 && ++num >= maxRows)
+                        break;
+                }
+            }
+            return caliTrainer.FinishTraining(ch);
+        }
+
         /// <summary>
         /// Trains a calibrator.
         /// </summary>
@@ -857,60 +937,14 @@ public static ICalibrator TrainCalibrator(IHostEnvironment env, IChannel ch, ICa
         {
             Contracts.CheckValue(env, nameof(env));
             env.CheckValue(ch, nameof(ch));
+            ch.CheckValue(caliTrainer, nameof(caliTrainer));
             ch.CheckValue(predictor, nameof(predictor));
             ch.CheckValue(data, nameof(data));
             ch.CheckParam(data.Schema.Label.HasValue, nameof(data), "data must have a Label column");
 
             var scored = ScoreUtils.GetScorer(predictor, data, env, null);
-
-            if (caliTrainer.NeedsTraining)
-            {
-                int labelCol;
-                if (!scored.Schema.TryGetColumnIndex(data.Schema.Label.Value.Name, out labelCol))
-                    throw ch.Except("No label column found");
-                int scoreCol;
-                if (!scored.Schema.TryGetColumnIndex(MetadataUtils.Const.ScoreValueKind.Score, out scoreCol))
-                    throw ch.Except("No score column found");
-                int weightCol = -1;
-                if (data.Schema.Weight?.Name is string weightName && scored.Schema.GetColumnOrNull(weightName)?.Index is int weightIdx)
-                    weightCol = weightIdx;
-                ch.Info("Training calibrator.");
-
-                var cols = weightCol > -1 ?
-                    new Schema.Column[] { scored.Schema[labelCol], scored.Schema[scoreCol], scored.Schema[weightCol] } :
-                    new Schema.Column[] { scored.Schema[labelCol], scored.Schema[scoreCol] };
-
-                using (var cursor = scored.GetRowCursor(cols))
-                {
-                    var labelGetter = RowCursorUtils.GetLabelGetter(cursor, labelCol);
-                    var scoreGetter = RowCursorUtils.GetGetterAs<Single>(NumberType.R4, cursor, scoreCol);
-                    ValueGetter<Single> weightGetter = weightCol == -1 ? (ref float dst) => dst = 1 :
-                        RowCursorUtils.GetGetterAs<Single>(NumberType.R4, cursor, weightCol);
-
-                    int num = 0;
-                    while (cursor.MoveNext())
-                    {
-                        Single label = 0;
-                        labelGetter(ref label);
-                        if (!FloatUtils.IsFinite(label))
-                            continue;
-                        Single score = 0;
-                        scoreGetter(ref score);
-                        if (!FloatUtils.IsFinite(score))
-                            continue;
-                        Single weight = 0;
-                        weightGetter(ref weight);
-                        if (!FloatUtils.IsFinite(weight))
-                            continue;
-
-                        caliTrainer.ProcessTrainingExample(score, label > 0, weight);
-
-                        if (maxRows > 0 && ++num >= maxRows)
-                            break;
-                    }
-                }
-            }
-            return caliTrainer.FinishTraining(ch);
+            var scoreColumn = scored.Schema[DefaultColumnNames.Score];
+            return TrainCalibrator(env, ch, caliTrainer, scored, data.Schema.Label.Value.Name, DefaultColumnNames.Score, data.Schema.Weight?.Name, maxRows);
         }
 
         public static IPredictorProducing<float> CreateCalibratedPredictor<TSubPredictor, TCalibrator>(IHostEnvironment env, TSubPredictor predictor, TCalibrator cali)
@@ -953,7 +987,8 @@ public ICalibratorTrainer CreateComponent(IHostEnvironment env)
     /// The probability of belonging to a particular class, for example class 1, is the number of class 1 instances in the bin, divided by the total number
     /// of instances in that bin.
     /// </summary>
-    public sealed class NaiveCalibratorTrainer : ICalibratorTrainer
+    [BestFriend]
+    internal sealed class NaiveCalibratorTrainer : ICalibratorTrainer
     {
         private readonly IHost _host;
 
@@ -1181,7 +1216,8 @@ public string GetSummary()
     /// <summary>
     /// Base class for calibrator trainers.
     /// </summary>
-    public abstract class CalibratorTrainerBase : ICalibratorTrainer
+    [BestFriend]
+    internal abstract class CalibratorTrainerBase : ICalibratorTrainer
     {
         protected readonly IHost Host;
         protected CalibrationDataStore Data;
@@ -1230,7 +1266,8 @@ public ICalibratorTrainer CreateComponent(IHostEnvironment env)
         }
     }
 
-    public sealed class PlattCalibratorTrainer : CalibratorTrainerBase
+    [BestFriend]
+    internal sealed class PlattCalibratorTrainer : CalibratorTrainerBase
     {
         internal const string UserName = "Sigmoid Calibration";
         internal const string LoadName = "PlattCalibration";
@@ -1389,7 +1426,8 @@ public override ICalibrator CreateCalibrator(IChannel ch)
         }
     }
 
-    public sealed class FixedPlattCalibratorTrainer : ICalibratorTrainer
+    [BestFriend]
+    internal sealed class FixedPlattCalibratorTrainer : ICalibratorTrainer
     {
         [TlcModule.Component(Name = "FixedPlattCalibrator", FriendlyName = "Fixed Platt Calibrator", Aliases = new[] { "FixedPlatt", "FixedSigmoid" })]
         public sealed class Arguments : ICalibratorTrainerFactory
@@ -1591,7 +1629,8 @@ public ICalibratorTrainer CreateComponent(IHostEnvironment env)
         }
     }
 
-    public class PavCalibratorTrainer : CalibratorTrainerBase
+    [BestFriend]
+    internal sealed class PavCalibratorTrainer : CalibratorTrainerBase
     {
         // a piece of the piecwise function
         private readonly struct Piece