'Compute matrix of (negated) sq distances.
Args:
boxlist1: BoxList holding N boxes.
boxlist2: BoxList holding M boxes.
Returns:
A tensor with shape [N, M] representing negated pairwise squared distance.'

def _compare(self, boxlist1, boxlist2):

    return ((-1) * box_list_ops.sq_dist(boxlist1, boxlist2))
