Recreate workspace directory when fit() is called, fix documentation, free up unmanaged memory. (dotnet#4438)

* Recreate workspace directory when fit() is called, fix documentation, free up unmanaged memory.

* PR feedback.

Author: codemzs

src/Microsoft.ML.Vision/ImageClassificationTrainer.cs

@@ -66,7 +66,8 @@ namespace Microsoft.ML.Vision
     ///
     /// ### Training Algorithm Details
     /// Trains a Deep Neural Network(DNN) by leveraging an existing pre-trained model such as Resnet50 for the purpose
-    /// of classifying images.
+    /// of classifying images. The technique was inspired from [TensorFlow's retrain image classification tutorial]
+    /// (https://www.tensorflow.org/hub/tutorials/image_retraining)
     /// ]]>
     /// </format>
     /// </remarks>
@@ -392,7 +393,7 @@ public sealed class Options : TrainerInputBaseWithLabel
             public Action<ImageClassificationMetrics> MetricsCallback = null;
 
             /// <summary>
-            /// Indicates the path where the models get downloaded to and cache files saved, default is a new temporary directory
+            /// Indicates the path where the image bottleneck cache files and trained model are saved, default is a new temporary directory
             /// </summary>
             [Argument(ArgumentType.AtMostOnce, HelpText = "Indicates the path where the models get downloaded to and cache files saved, default is a new temporary directory.", SortOrder = 15)]
             public string WorkspacePath = null;
@@ -591,6 +592,7 @@ private void InitializeTrainingGraph(IDataView input)
             _classCount = labelCount == 1 ? 2 : (int)labelCount;
             var imageSize = ImagePreprocessingSize[_options.Arch];
             _session = LoadTensorFlowSessionFromMetaGraph(Host, _options.Arch).Session;
+            _session.graph.as_default();
             (_jpegData, _resizedImage) = AddJpegDecoding(imageSize.Item1, imageSize.Item2, 3);
             _jpegDataTensorName = _jpegData.name;
             _resizedImageTensorName = _resizedImage.name;
@@ -631,6 +633,14 @@ private protected override MulticlassPredictionTransformer<ImageClassificationMo
 
         private protected override ImageClassificationModelParameters TrainModelCore(TrainContext trainContext)
         {
+            // Workspace directory is cleaned after training run. However, the pipeline can be re-used by calling
+            // fit() again after transform(), in which case we must ensure workspace directory exists. This scenario
+            // is typical in the case of cross-validation.
+            if (!Directory.Exists(_options.WorkspacePath))
+            {
+                Directory.CreateDirectory(_options.WorkspacePath);
+            }
+
             InitializeTrainingGraph(trainContext.TrainingSet.Data);
             CheckTrainingParameters(_options);
             var validationSet = trainContext.ValidationSet?.Data ?? _options.ValidationSet;
@@ -1301,7 +1311,7 @@ private void VariableSummaries(RefVariable var)
                 var optimizer = useLearningRateScheduler ? tf.train.GradientDescentOptimizer(_learningRateInput) :
                                     tf.train.GradientDescentOptimizer(learningRate);
 
-                _trainStep = optimizer.minimize(crossEntropyMean);
+               _trainStep = optimizer.minimize(crossEntropyMean);
             });
 
             return (_trainStep, crossEntropyMean, _labelTensor, _softMaxTensor);
@@ -1341,6 +1351,11 @@ private void Dispose(bool disposing)
             {
                 _session.close();
             }
+
+            if (_session != null && _session.graph != IntPtr.Zero)
+            {
+                _session.graph.Dispose();
+            }
         }
 
         /// <summary>
@@ -1527,6 +1542,11 @@ private void Dispose(bool disposing)
             {
                 _session.close();
             }
+
+            if (_session != null && _session.graph != IntPtr.Zero)
+            {
+                _session.graph.Dispose();
+            }
         }
     }
 }