code: def add_optional_arg_param(self, param_name, layer_index, blob_index):
          
          blobs = self.layers[layer_index].blobs
          if blob_index < len(blobs):
              self.add_arg_param(param_name, layer_index, blob_index)
docs: Add an arg param. If there is no such param in .caffemodel fie, silently ignore it.