issue 2954: c# API memory leak

bindings/csharp/CNTKLibraryManagedDll/ShimApiClasses/TrainerShim.cs

@@ -22,7 +22,7 @@ namespace CNTK
         public static Trainer CreateTrainer(Function model, Function lossFunction, Function evaluationFunction, IList<Learner> parameterLearners, 
             ProgressWriterVector progressWriters = null)
         {
-            LearnerVector learnerVector = Helper.AsLearnerVector(parameterLearners);
+            using (LearnerVector learnerVector = Helper.AsLearnerVector(parameterLearners))
                 if (progressWriters != null)
                     return CNTKLib.CreateTrainer(model, lossFunction, evaluationFunction, learnerVector, progressWriters);
                 else
@@ -37,7 +37,7 @@ namespace CNTK
         /// <returns></returns>
         public bool TrainMinibatch(IDictionary<Variable, MinibatchData> arguments, DeviceDescriptor computeDevice)
         {
-            UnorderedMapVariableMinibatchData vectorData = Helper.AsUnorderedMapVariableMinibatchData(arguments);
+            using (UnorderedMapVariableMinibatchData vectorData = Helper.AsUnorderedMapVariableMinibatchData(arguments))
                 return _TrainMinibatch(vectorData, computeDevice);
         }
 
@@ -50,8 +50,8 @@ namespace CNTK
         [System.Obsolete("TrainMinibatch() without isSweepEndInarguments will be deprecated soon. Please TrainMinibatch() with isSweepEndInarguments.", false)]
         public bool TrainMinibatch(IDictionary<Variable, Value> arguments, DeviceDescriptor computeDevice)
         {
-            UnorderedMapVariableValuePtr mapData = Helper.AsUnorderedMapVariableValue(arguments);
             bool isSweepEndInarguments = false;
+            using (UnorderedMapVariableValuePtr mapData = Helper.AsUnorderedMapVariableValue(arguments))
                 return _TrainMinibatch(mapData, isSweepEndInarguments, computeDevice);
         }
 
@@ -64,7 +64,7 @@ namespace CNTK
         /// <returns></returns>
         public bool TrainMinibatch(IDictionary<Variable, Value> arguments, bool isSweepEndInarguments, DeviceDescriptor computeDevice)
         {
-            UnorderedMapVariableValuePtr mapData = Helper.AsUnorderedMapVariableValue(arguments);
+            using (UnorderedMapVariableValuePtr mapData = Helper.AsUnorderedMapVariableValue(arguments))
                 return _TrainMinibatch(mapData, isSweepEndInarguments, computeDevice);
         }
     }