MPI Model Averaging. Added check to short-circuit empty minibatch

MachineLearning/cn/SGD.h

@@ -1040,6 +1040,8 @@ namespace Microsoft { namespace MSR { namespace CNTK {
                 UpdateEvalTimeStamps(labelNodes);
 
                 size_t actualMBSize = net.GetActualMBSize();
+                if (0 == actualMBSize)
+                    continue;
 
                 net.SetActualMiniBatchSize(actualMBSize);
                 net.SetActualNbrSlicesInEachRecIter(trainSetDataReader->NumberSlicesInEachRecurrentIter());