Changed to avoid compiler warning.

examples/dnn_semantic_segmentation_train_ex.cpp

@@ -279,7 +279,7 @@ int main(int argc, char** argv) try
     }
 
     // a mini-batch smaller than the default can be used with GPUs having less memory
-    const int minibatch_size = argc == 3 ? std::stoi(argv[2]) : 23;
+    const unsigned int minibatch_size = argc == 3 ? std::stoi(argv[2]) : 23;
     cout << "mini-batch size: " << minibatch_size << endl;
 
     const double initial_learning_rate = 0.1;