MMdetection unable to form final test file

I am trying to use MMdetection 2.0 for MASKD object detection. However, I am facing difficulty in creating the test file.
Here is the code that I have written

from mmdet.datasets import build_dataloader = True
distributed = False
val_dataset = build_dataset(
data_loader = build_dataloader(
from mmcv.runner import load_checkpoint
from mmcv.parallel import MMDataParallel, MMDistributedDataParallel
from mmdet.apis import single_gpu_test
device = torch.device('cuda') if torch.cuda.is_available() else torch.device('cpu')

model = build_detector(cfg.model, train_cfg=None, test_cfg=cfg.test_cfg)
checkpoint = load_checkpoint(model, WEIGHTS_FILE, map_location='cpu')

model.CLASSES = dataset.CLASSES

model = MMDataParallel(model, device_ids=[0])
outputs = single_gpu_test(model, data_loader, False, None, 0.5)

However I get the following output

IndexError                                Traceback (most recent call last)
<ipython-input-122-4158962a1af4> in <module>()
----> 1 val_dataset.format_results(outputs)

2 frames
/content/mmdetection/mmdet/datasets/ in format_results(self, results, jsonfile_prefix, **kwargs)
    359         else:
    360             tmp_dir = None
--> 361         result_files = self.results2json(results, jsonfile_prefix)
    362         return result_files, tmp_dir

/content/mmdetection/mmdet/datasets/ in results2json(self, results, outfile_prefix)
    291         result_files = dict()
    292         if isinstance(results[0], list):
--> 293             json_results = self._det2json(results)
    294             result_files['bbox'] = f'{outfile_prefix}.bbox.json'
    295             result_files['proposal'] = f'{outfile_prefix}.bbox.json'

/content/mmdetection/mmdet/datasets/ in _det2json(self, results)
    228                     data['bbox'] = self.xyxy2xywh(bboxes[i])
    229                     data['score'] = float(bboxes[i][4])
--> 230                     data['category_id'] = self.cat_ids[label]
    231                     json_results.append(data)
    232         return json_results

IndexError: list index out of range

I guess I am unable to get the category_id but I cant find how to fix that.
Please help

Looking at the starter/baseline code. There’s the example for preparing the submission.