fix test_integration_tests_coco

Summary:
Pull Request resolved: https://github.com/facebookresearch/d2go/pull/671

> AttributeError: type object 'GeneralizedRCNNTask' has no attribute 'cleanup'

EZ fix

Reviewed By: rbasch

Differential Revision: D60400187

fbshipit-source-id: 25872f4928cf8851ff63e96311c12086e272d619

d2go/runner/lightning_task.py

@@ -369,6 +369,10 @@ class DefaultTask(D2GoDataAPIMixIn, pl.LightningModule):
             cfg=cfg, dataset_name=dataset_name, output_folder=output_folder
         )
 
+    @classmethod
+    def cleanup(cls) -> None:
+        pass
+
     # ---------------------------------------------------------------------------
     # Hooks
     # ---------------------------------------------------------------------------