message: add support for   tf.add_n operation
diff: @@ -942,7 +942,7 @@ void TFImporter::populateNet(Net dstNet)
  if (getDataLayout(name, data_layouts) == DATA_LAYOUT_UNKNOWN)
  data_layouts[name] = DATA_LAYOUT_NHWC;
  }
  - else if (type == "BiasAdd" || type == "Add" || type == "Sub")
  + else if (type == "BiasAdd" || type == "Add" || type == "Sub" || type=="AddN")
  {
  bool haveConst = false;
  for(int ii = 0; !haveConst && ii < layer.input_size(); ++ii)
  