[Fix] Fixed bug on Custom3DDataset (#1519)

* Fixed bug on Custom3DDataset

The func 'get_ann_info' was supposed to return a dict with the following keys:
- gt_bboxes_3d
- gt_labels_3d
- gt_names
but gt_names wasn't included in the return dict.

I added this key-value couple to the return dict of 'get_ann_info'

* fixed code linting

mmdet3d/datasets/custom_3d.py

@@ -182,7 +182,9 @@ class Custom3DDataset(Dataset):
             origin=(0.5, 0.5, 0.5)).convert_to(self.box_mode_3d)
 
         anns_results = dict(
-            gt_bboxes_3d=gt_bboxes_3d, gt_labels_3d=gt_labels_3d)
+            gt_bboxes_3d=gt_bboxes_3d,
+            gt_labels_3d=gt_labels_3d,
+            gt_names=gt_names_3d)
         return anns_results
 
     def pre_pipeline(self, results):