exposing the biases for multi-class logistic regression (dotnet#1224)

* exposing the biases for multi-class logistic regression

* Adding the GetBiases to the cookbook samples.

docs/code/MlNetCookBook.md

@@ -617,6 +617,9 @@ var model = learningPipeline.Fit(trainData);
 VBuffer<float>[] weights = null;
 predictor.GetWeights(ref weights, out int numClasses);
 
+// similarly we can also inspect the biases for the 3 classes
+var biases = pred.GetBiases();
+
 // Inspect the normalizer scales.
 Console.WriteLine(string.Join(" ", normScales));
 ```

src/Microsoft.ML.StandardLearners/Standard/LogisticRegression/MulticlassLogisticRegression.cs

@@ -232,7 +232,7 @@ protected override VBuffer<float> InitializeWeightsFromPredictor(MulticlassLogis
             if (srcPredictor.InputType.VectorSize != NumFeatures)
                 throw Contracts.Except("The input training data must have the same features used to train the input predictor.");
 
-            return InitializeWeights(srcPredictor.DenseWeightsEnumerable(), srcPredictor.BiasesEnumerable());
+            return InitializeWeights(srcPredictor.DenseWeightsEnumerable(), srcPredictor.GetBiases());
         }
 
         protected override MulticlassLogisticRegressionPredictor CreatePredictor()
@@ -952,7 +952,10 @@ internal IEnumerable<float> DenseWeightsEnumerable()
             }
         }
 
-        internal IEnumerable<float> BiasesEnumerable()
+        /// <summary>
+        /// Gets the biases for the logistic regression predictor.
+        /// </summary>
+        public IEnumerable<float> GetBiases()
         {
             return _biases;
         }

test/Microsoft.ML.StaticPipelineTesting/Training.cs

@@ -289,6 +289,9 @@ public void SdcaMulticlass()
             foreach (var w in weights)
                 Assert.True(w.Length == 4);
 
+            var biases = pred.GetBiases();
+            Assert.True(biases.Count() == 3);
+
             var data = model.Read(dataSource);
 
             // Just output some data on the schema for fun.

test/Microsoft.ML.Tests/Scenarios/Api/CookbookSamples/CookbookSamples.cs

@@ -292,6 +292,9 @@ private void TrainAndInspectWeights(string dataPath)
             VBuffer<float>[] weights = null;
             predictor.GetWeights(ref weights, out int numClasses);
 
+            // similarly we can also inspect the biases for the 3 classes 
+            var biases = predictor.GetBiases();
+
             // Inspect the normalizer scales.
             Console.WriteLine(string.Join(" ", normScales));
         }