Args:
            boxes: Nx4
        Returns:
            BoxProposals

def match_box_with_gt(self, boxes, iou_threshold):
        """
        Args:
            boxes: Nx4
        Returns:
            BoxProposals
        """
        if self.is_training:
            with tf.name_scope('match_box_with_gt_{}'.format(iou_threshold)):
                iou = pairwise_iou(boxes, self.gt_boxes)  # NxM
                max_iou_per_box = tf.reduce_max(iou, axis=1)  # N
                best_iou_ind = tf.argmax(iou, axis=1)  # N
                labels_per_box = tf.gather(self.gt_labels, best_iou_ind)
                fg_mask = max_iou_per_box >= iou_threshold
                fg_inds_wrt_gt = tf.boolean_mask(best_iou_ind, fg_mask)
                labels_per_box = tf.stop_gradient(labels_per_box * tf.cast(fg_mask, tf.int64))
                return BoxProposals(boxes, labels_per_box, fg_inds_wrt_gt)
        else:
            return BoxProposals(boxes)