Slice anchors to the spatial size of this featuremap.

def narrow_to(self, featuremap):
        """
        Slice anchors to the spatial size of this featuremap.
        """
        shape2d = tf.shape(featuremap)[2:]  # h,w
        slice3d = tf.concat([shape2d, [-1]], axis=0)
        slice4d = tf.concat([shape2d, [-1, -1]], axis=0)
        boxes = tf.slice(self.boxes, [0, 0, 0, 0], slice4d)
        gt_labels = tf.slice(self.gt_labels, [0, 0, 0], slice3d)
        gt_boxes = tf.slice(self.gt_boxes, [0, 0, 0, 0], slice4d)
        return RPNAnchors(boxes, gt_labels, gt_boxes)