Args:
      boxes: nx4 floatbox

    Returns:
      n

def area(boxes):
    """
    Args:
      boxes: nx4 floatbox

    Returns:
      n
    """
    x_min, y_min, x_max, y_max = tf.split(boxes, 4, axis=1)
    return tf.squeeze((y_max - y_min) * (x_max - x_min), [1])