title: detectron2 training keyerror
body: <p>I am trying to train my own COCO dataset with detectron2, however when I start my own training I encounter a key error</p>
  <blockquote>
  <p>KeyError: 'category_id</p>
  </blockquote>
  <pre><code>error code : https://i.stack.imgur.com/yO5IO.png
  
  //this is the code i am training with 
  from detectron2.engine import DefaultTrainer
  from detectron2.config import get_cfg
  import os
  
  cfg = get_cfg()
  cfg.merge_from_file(model_zoo.get_config_file(&quot;COCO- 
  InstanceSegmentation/mask_rcnn_R_50_FPN_3x.yaml&quot;))
  cfg.DATASETS.TRAIN = (&quot;coco_train_new&quot;,)
  cfg.DATASETS.TEST = ()
  cfg.DATALOADER.NUM_WORKERS = 2
  cfg.MODEL.WEIGHTS = &quot;detectron2://COCO-InstanceSegmentation/mask_rcnn_R_50_FPN_3x/137849600/model_final_f10217.pkl&quot;  
  cfg.SOLVER.IMS_PER_BATCH = 2
  cfg.SOLVER.BASE_LR = 0.00025  # pick a good LR
  cfg.SOLVER.MAX_ITER = 1000    # 300 iterations seems good enough for this toy dataset; you may need to train longer for a practical dataset
  cfg.MODEL.ROI_HEADS.BATCH_SIZE_PER_IMAGE = 512   # faster, and good enough for this toy dataset (default: 512)
  cfg.MODEL.ROI_HEADS.NUM_CLASSES = 1  # only has one class (ballon)
  
  os.makedirs(cfg.OUTPUT_DIR, exist_ok=True)
  trainer = DefaultTrainer(cfg) 
  trainer.resume_or_load(resume=False)
  trainer.train()
  </code></pre>
  <p>so I went back to check on my COCO json file, however the json file was output with standard COCO format,
  any idea what might cause the problem</p>
  <p>p.s I am training the data with the detectron2 sample code, so I think there shouldn't be a problem</p>