fix dbsampler bug (#1812)

mmdet3d/datasets/transforms/transforms_3d.py

@@ -367,11 +367,10 @@ class ObjectSample(BaseTransform):
         gt_bboxes_3d = input_dict['gt_bboxes_3d']
         gt_labels_3d = input_dict['gt_labels_3d']
 
-        if self.use_ground_plane and 'plane' in input_dict['ann_info']:
-            ground_plane = input_dict['plane']
+        if self.use_ground_plane:
+            ground_plane = input_dict.get('plane', None)
             assert ground_plane is not None, '`use_ground_plane` is True ' \
                                              'but find plane is None'
-            input_dict['plane'] = ground_plane
         else:
             ground_plane = None
         # change to float for blending operation