inst_a = tf.constant(
        [[[1, 2, 3, 4], [5, 6, 7, 8], [-1, -1, -1, -1], [-1, -1, -1, -1]],
         [[-1, -1, -1, -1], [-1, -1, -1, -1], [4, 3, 2, 1], [8, 7, 6, 5]],
         [[1, 2, 3, 4], [5, 6, 7, 8], [4, 3, 2, 1], [8, 7, 6, 5]]],
        dtype=tf.float32)