'append flipped images to an roidb
flip boxes coordinates, images will be actually flipped when loading into network
:param roidb: [image_index][\'boxes\', \'gt_classes\', \'gt_overlaps\', \'flipped\']
:return: roidb: [image_index][\'boxes\', \'gt_classes\', \'gt_overlaps\', \'flipped\']'

def append_flipped_images(self, roidb):

    logger.info(('%s    append    flipped    images    to    roidb' % self.name))
    assert (self.num_images == len(roidb))
    for i in range(self.num_images):
        roi_rec = roidb[i]
        boxes = roi_rec['boxes'].copy()
        oldx1 = boxes[:, 0].copy()
        oldx2 = boxes[:, 2].copy()
        boxes[:, 0] = ((roi_rec['width'] - oldx2) - 1)
        boxes[:, 2] = ((roi_rec['width'] - oldx1) - 1)
        assert (boxes[:, 2] >= boxes[:, 0]).all()
        entry = {'image': roi_rec['image'], 'height': roi_rec['height'], 'width': roi_rec['width'], 'boxes': boxes, 'gt_classes': roidb[i]['gt_classes'], 'gt_overlaps': roidb[i]['gt_overlaps'], 'max_classes': roidb[i]['max_classes'], 'max_overlaps': roidb[i]['max_overlaps'], 'flipped': True}
        roidb.append(entry)
    self.image_set_index *= 2
    return roidb
