Fix index variable in for loop in paddlestructure.cpp

[JIANG3330]

PaddleStructure::rebuild_table函数中structure_boxes的索引用错了，可能导致dis和iou无法正确计算。
原代码段：

    for (int j = 0; j < structure_boxes.size(); j++) {
      if (structure_boxes[i].size() == 8) {
        structure_box = Utility::xyxyxyxy2xyxy(structure_boxes[j]);
      } else {
        structure_box = structure_boxes[j];
      }
      dis_list[j][0] = this->dis(ocr_box, structure_box);
      dis_list[j][1] = 1 - Utility::iou(ocr_box, structure_box);
      dis_list[j][2] = j;
    }

应改为：

    for (int j = 0; j < structure_boxes.size(); j++) {
      if (structure_boxes[j].size() == 8) {
        structure_box = Utility::xyxyxyxy2xyxy(structure_boxes[j]);
      } else {
        structure_box = structure_boxes[j];
      }
      dis_list[j][0] = this->dis(ocr_box, structure_box);
      dis_list[j][1] = 1 - Utility::iou(ocr_box, structure_box);
      dis_list[j][2] = j;
    }

[paddle-bot]

Thanks for your contribution!

[andyjiang1116]

@JIANG3330 感谢修复，麻烦将这个bug同步修复到dygraph分支吧