code: def bbox_flip(bbox, width, flip_x=False):
      
      if flip_x:
          xmax = width - bbox[:, 0]
          xmin = width - bbox[:, 2]
          bbox[:, 0] = xmin
          bbox[:, 2] = xmax
      return bbox
docs: invalid value in bbox_transform if this wrong (no overlap), note index 0 and 2
      also note need to save before assignment
      :param bbox: [n][x1, y1, x2, y2]
      :param width: cv2 (height, width, channel)
      :param flip_x: will flip x1 and x2
      :return: flipped box