损失函数计算部分代码的疑问(get_target中)

[nothasson]

            # -------------------------------------------------------#
            #   将真实框转换一个形式
            #   num_true_box, 4
            # -------------------------------------------------------#
            gt_box = torch.FloatTensor(torch.cat((torch.zeros((batch_target.size(0), 2)), batch_target[:, 2:4]), 1))
            # -------------------------------------------------------#
            #   将先验框转换一个形式
            #   9, 4
            # -------------------------------------------------------#
            anchor_shapes = torch.FloatTensor(
                torch.cat((torch.zeros((len(anchors), 2)), torch.FloatTensor(anchors)), 1))
            # -------------------------------------------------------#
            #   计算交并比
            #   self.calculate_iou(gt_box, anchor_shapes) = [num_true_box, 9]每一个真实框和9个先验框的重合情况
            #   best_ns:
            #   [每个真实框最大的重合度max_iou, 每一个真实框最重合的先验框的序号]
            # -------------------------------------------------------#
            best_ns = torch.argmax(self.calculate_iou(gt_box, anchor_shapes), dim=-1)

请问一下，为什么gt_box和anchor_shapes的前两位元素要设为0,0呀？