Fix memory leak in TensorflowTransform (dotnet#4223)

* Fix memory leak by freeing input tensors after executing the graph

* Fix memory leak by freeing the graph after each session

* Get input tensors from runner

Author: KsenijaS

src/Microsoft.ML.Dnn/DnnUtils.cs

@@ -399,6 +399,11 @@ public Runner AddInput(Tensor value, int index)
                 return this;
             }
 
+            public List<Tensor> GetInputValues()
+            {
+                return _inputValues;
+            }
+
             public Runner AddOutputs(string output)
             {
                 _outputs.Add(ParseOutput(output));

src/Microsoft.ML.TensorFlow/TensorflowTransform.cs

@@ -463,6 +463,11 @@ private void Dispose(bool disposing)
                 {
                     Session.close(); // invoked Dispose()
                 }
+
+                if (Session != null && Session.graph != IntPtr.Zero)
+                {
+                    Session.graph.Dispose();
+                }
             }
             finally
             {
@@ -645,7 +650,7 @@ private void UpdateCacheIfNeeded(long position, ITensorValueGetter[] srcTensorGe
                     Runner runner = new Runner(_parent.Session);
 
                     // Feed inputs to the graph.
-                    for (int i = 0; i < _parent.Inputs.Length; i++)
+                     for (int i = 0; i < _parent.Inputs.Length; i++)
                     {
                         var tensor = srcTensorGetters[i].GetTensor();
                         runner.AddInput(_parent.Inputs[i], tensor);
@@ -658,6 +663,12 @@ private void UpdateCacheIfNeeded(long position, ITensorValueGetter[] srcTensorGe
                     // Execute the graph.
                     var tensors = runner.Run();
 
+                    List<Tensor> inputTensors = runner.GetInputValues();
+                    foreach (Tensor inputTensor in inputTensors)
+                    {
+                        inputTensor.Dispose();
+                    }
+
                     Contracts.Assert(tensors.Length > 0);
 
                     for (int j = 0; j < activeOutputColNames.Length; j++)