Computes pairwise intersection-over-union between box collections.

    Args:
      boxlist1: Nx4 floatbox
      boxlist2: Mx4

    Returns:
      a tensor with shape [N, M] representing pairwise iou scores.

def pairwise_iou(boxlist1, boxlist2):
    """Computes pairwise intersection-over-union between box collections.

    Args:
      boxlist1: Nx4 floatbox
      boxlist2: Mx4

    Returns:
      a tensor with shape [N, M] representing pairwise iou scores.
    """
    intersections = pairwise_intersection(boxlist1, boxlist2)
    areas1 = area(boxlist1)
    areas2 = area(boxlist2)
    unions = (
        tf.expand_dims(areas1, 1) + tf.expand_dims(areas2, 0) - intersections)
    return tf.where(
        tf.equal(intersections, 0.0),
        tf.zeros_like(intersections), tf.truediv(intersections, unions))