Defaults for ImageClassification API (#4415)

* Changed some defaults

* Changed metrics callback default

* metricsCallback will write to mlcontext log by default. The sample has been update to show how to get the output to console from the log.

* deleted unnecessary comments

* Addressed comments

* Minor clean up.

* Disable unstable test.

Author: harshithapv

docs/samples/Microsoft.ML.Samples/Dynamic/Trainers/MulticlassClassification/ImageClassification/ImageClassificationDefault.cs

@@ -29,13 +29,15 @@ public static void Example()
             //Download the image set and unzip
             string finalImagesFolderName = DownloadImageSet(
                 imagesDownloadFolderPath);
+
             string fullImagesetFolderPath = Path.Combine(
                 imagesDownloadFolderPath, finalImagesFolderName);
 
             try
             {
 
                 MLContext mlContext = new MLContext(seed: 1);
+                mlContext.Log += MlContext_Log;
 
                 //Load all the original images info
                 IEnumerable<ImageData> images = LoadImagesFromDirectory(
@@ -60,7 +62,7 @@ public static void Example()
                 IDataView testDataset = trainTestData.TestSet;
 
                 var pipeline = mlContext.MulticlassClassification.Trainers
-                        .ImageClassification(featureColumnName:"Image", validationSet:testDataset)
+                        .ImageClassification(featureColumnName:"Image")
                     .Append(mlContext.Transforms.Conversion.MapKeyToValue(outputColumnName: "PredictedLabel",
                         inputColumnName: "PredictedLabel"));
 
@@ -109,6 +111,14 @@ public static void Example()
             Console.ReadKey();
         }
 
+        private static void MlContext_Log(object sender, LoggingEventArgs e)
+        {
+            if (e.Message.StartsWith("[Source=ImageClassificationTrainer;"))
+            {
+                Console.WriteLine(e.Message);
+            }
+        }
+
         private static void TrySinglePrediction(string imagesForPredictions,
             MLContext mlContext, ITransformer trainedModel)
         {

src/Microsoft.ML.Vision/ImageClassificationTrainer.cs

@@ -359,7 +359,7 @@ public sealed class Options : TrainerInputBaseWithLabel
             /// Early stopping technique parameters to be used to terminate training when training metric stops improving.
             /// </summary>
             [Argument(ArgumentType.AtMostOnce, HelpText = "Early stopping technique parameters to be used to terminate training when training metric stops improving.", SortOrder = 15)]
-            public EarlyStopping EarlyStoppingCriteria;
+            public EarlyStopping EarlyStoppingCriteria = new EarlyStopping();
 
             /// <summary>
             /// Specifies the model architecture to be used in the case of image classification training using transfer learning.
@@ -437,7 +437,7 @@ public sealed class Options : TrainerInputBaseWithLabel
             /// A class that performs learning rate scheduling.
             /// </summary>
             [Argument(ArgumentType.AtMostOnce, HelpText = "A class that performs learning rate scheduling.", SortOrder = 15)]
-            public LearningRateScheduler LearningRateScheduler = new LsrDecay();
+            public LearningRateScheduler LearningRateScheduler = new ExponentialLRDecay();
         }
 
         /// <summary> Return the type of prediction task.</summary>
@@ -532,6 +532,12 @@ internal ImageClassificationTrainer(IHostEnvironment env, Options options)
                 options.ValidationSetBottleneckCachedValuesFileName = _options.ValidationSetBottleneckCachedValuesFileName;
             }
 
+            if (options.MetricsCallback == null)
+            {
+                var logger = Host.Start(nameof(ImageClassificationTrainer));
+                options.MetricsCallback = (ImageClassificationMetrics metric) => { logger.Trace(metric.ToString()); };
+            }
+
             _options = options;
             _useLRScheduling = _options.LearningRateScheduler != null;
             _checkpointPath = Path.Combine(_options.WorkspacePath, _options.FinalModelPrefix +

test/Microsoft.ML.Tests/ScenariosWithDirectInstantiation/TensorflowTests.cs

@@ -1431,7 +1431,7 @@ public void TensorFlowImageClassificationWithExponentialLRScheduling()
             TensorFlowImageClassificationWithLRScheduling(new ExponentialLRDecay());
         }
 
-        [TensorFlowFact]
+        [Fact(Skip ="Very unstable tests, causing many build failures.")]
         public void TensorFlowImageClassificationWithPolynomialLRScheduling()
         {
             TensorFlowImageClassificationWithLRScheduling(new PolynomialLRDecay());