fix dp extraction bug for visualize(). (#934)

* fix dp extraction bug for visualize().

* gt_masks is actually not needed here.

examples/FasterRCNN/train.py

@@ -349,11 +349,7 @@ def visualize(model, model_path, nr_visualize=100, output_dir='output'):
     utils.fs.mkdir_p(output_dir)
     with tqdm.tqdm(total=nr_visualize) as pbar:
         for idx, dp in itertools.islice(enumerate(df), nr_visualize):
-            img = dp[0]
-            if cfg.MODE_MASK:
-                gt_boxes, gt_labels, gt_masks = dp[-3:]
-            else:
-                gt_boxes, gt_labels = dp[-2:]
+            img, gt_boxes, gt_labels = dp['image'], dp['gt_boxes'], dp['gt_labels']
 
             rpn_boxes, rpn_scores, all_scores, \
                 final_boxes, final_scores, final_labels = pred(img, gt_boxes, gt_labels)