'return ground truth image regions database
:return: imdb[image_index][\'boxes\', \'gt_classes\', \'gt_overlaps\', \'flipped\']'

def gt_roidb(self):

    cache_file = os.path.join(self.cache_path, (self.name + '_gt_roidb.pkl'))
    if os.path.exists(cache_file):
        with open(cache_file, 'rb') as fid:
            roidb = cPickle.load(fid)
        logger.info(('%s    gt    roidb    loaded    from    %s' % (self.name, cache_file)))
        return roidb
    gt_roidb = [self.load_pascal_annotation(index) for index in self.image_set_index]
    with open(cache_file, 'wb') as fid:
        cPickle.dump(gt_roidb, fid, cPickle.HIGHEST_PROTOCOL)
    logger.info(('%s    wrote    gt    roidb    to    %s' % (self.name, cache_file)))
    return gt_roidb
