allow overriding get_mapper

Summary:
Pull Request resolved: https://github.com/facebookresearch/d2go/pull/63

As titled.

Reviewed By: zhanghang1989

Differential Revision: D27945627

fbshipit-source-id: 58180ace6ab327a5eb025f3227ae6c22601a3580

d2go/runner/lightning_task.py

@@ -313,6 +313,10 @@ class DefaultTask(pl.LightningModule):
             cfg=cfg, dataset_name=dataset_name, output_folder=output_folder
         )
 
+    @staticmethod
+    def get_mapper(cfg, is_train):
+        return Detectron2GoRunner.get_mapper(cfg, is_train)
+
     @staticmethod
     def build_detection_train_loader(cfg, *args, mapper=None, **kwargs):
         return Detectron2GoRunner.build_detection_train_loader(cfg, *args, **kwargs)