gpu_id is None when args.use_gpu as False

tools/infer/predict_det.py

@@ -111,7 +111,7 @@ def __init__(self, args):
  inference_config=self.config,
  pids=pid,
  process_name=None,
- gpu_ids=gpu_id,
+ gpu_ids=gpu_id if args.use_gpu else None,
  time_keys=[
  'preprocess_time', 'inference_time', 'postprocess_time'
  ],