title: COCO api evaluation for subset of classes
body: <p>I'm using the python coco api to run evaluation for object detection. I have two files, a ground truth json, and a results json. The <a href="https://github.com/cocodataset/cocoapi/blob/master/PythonAPI/pycocoEvalDemo.ipynb" rel="noreferrer">coco notebook demo</a> only shows running eval for all classes. How can I run it for only one specific class or a subset of classes? Currently I'm doing this:</p>
  
  <pre><code>from pycocotools.coco import COCO
  from pycocotools.cocoeval import COCOeval
  
  annType = 'bbox'
  
  cocoGt=COCO(gt_json)
  cocoDt=cocoGt.loadRes(results_json)
  
  imgIds=sorted(cocoGt.getImgIds())
  
  # running evaluation
  cocoEval = COCOeval(cocoGt,cocoDt,annType)
  cocoEval.params.imgIds = imgIds
  cocoEval.evaluate()
  cocoEval.accumulate()
  cocoEval.summarize()
  </code></pre>