title: AttributeError: Cannot find field 'pred_masks' in the given Instances
body: <p>I trained a model with RetinaNet Detectron2 on colab. The training is successful but i am unable to test it on images.</p>
  <p>Code:</p>
  <pre><code>from detectron2.utils.visualizer import ColorMode
  import glob
  
  for imageName in glob.glob('/content/drive/My Drive/Detectron/test/*jpg'):
      print(imageName)
      im = cv2.imread(imageName)
      outputs = predictor(im)
      v = Visualizer(im[:, :, ::-1],
                  metadata=test_metadata, 
                  scale=0.8,
                  instance_mode=ColorMode.IMAGE_BW
                   )
      out = v.draw_instance_predictions(outputs[&quot;instances&quot;].to(&quot;cpu&quot;))
      cv2_imshow(out.get_image()[:, :, ::-1])
  </code></pre>
  <p>Error:</p>
  <blockquote>
  <p>/content/drive/My
  Drive/Detectron/test/wh_dresscode_4969_jpg.rf.5a2702518691d3d6f9cef7ad7977e5c8.jpg
  data type: &lt;class 'detectron2.structures.instances.Instances'&gt;
  AttributeError                            Traceback (most recent call
  last)</p>
  <p>AttributeError: Cannot find field 'pred_masks' in the given Instances!</p>
  </blockquote>