example results
        [{"image_id": 42,
          "category_id": 18,
          "bbox": [258.15,41.29,348.26,243.78],
          "score": 0.236}, ...]

def _write_coco_results(self, _coco, detections):
        """ example results
        [{"image_id": 42,
          "category_id": 18,
          "bbox": [258.15,41.29,348.26,243.78],
          "score": 0.236}, ...]
        """
        cats = [cat['name'] for cat in _coco.loadCats(_coco.getCatIds())]
        class_to_coco_ind = dict(zip(cats, _coco.getCatIds()))
        results = []
        for cls_ind, cls in enumerate(self.classes):
            if cls == '__background__':
                continue
            logger.info('collecting %s results (%d/%d)' % (cls, cls_ind, self.num_classes - 1))
            coco_cat_id = class_to_coco_ind[cls]
            results.extend(self._coco_results_one_category(detections[cls_ind], coco_cat_id))
        logger.info('writing results json to %s' % self._result_file)
        with open(self._result_file, 'w') as f:
            json.dump(results, f, sort_keys=True, indent=4)