Add a skip-connection to the descriptor.
        Args:
            u: Number of convolutional layers before the starting point.
            v: Number of convolutional layers before the ending point.
            connection_type: Must be either CONCAT_CONNECT or ADD_CONNECT.

def add_skip_connection(self, u, v, connection_type):
        """ Add a skip-connection to the descriptor.
        Args:
            u: Number of convolutional layers before the starting point.
            v: Number of convolutional layers before the ending point.
            connection_type: Must be either CONCAT_CONNECT or ADD_CONNECT.
        """
        if connection_type not in [self.CONCAT_CONNECT, self.ADD_CONNECT]:
            raise ValueError(
                "connection_type should be NetworkDescriptor.CONCAT_CONNECT "
                "or NetworkDescriptor.ADD_CONNECT."
            )
        self.skip_connections.append((u, v, connection_type))