code: def narrow_to(self, 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)
docs: Slice anchors to the spatial size of this featuremap.