fix import bug for oss

Summary:
Pull Request resolved: https://github.com/facebookresearch/d2go/pull/153

Fix the error:
'''
ImportError: cannot import name 'Detectron2GoRunner' from 'd2go.runner' (/home/runner/work/d2go/d2go/d2go/runner/__init__.py)
'''

Reviewed By: zhanghang1989

Differential Revision: D33306196

fbshipit-source-id: 6c773260855498ed498bd43f998bc4de891ff9e2

d2go/runner/__init__.py

@@ -7,7 +7,21 @@ from typing import Type, Union, Optional
 
 from pytorch_lightning import LightningModule
 
-from .default_runner import BaseRunner, GeneralizedRCNNRunner
+from .default_runner import (
+    BaseRunner,
+    Detectron2GoRunner,
+    GeneralizedRCNNRunner,
+    TRAINER_HOOKS_REGISTRY,
+)
+
+
+__all__ = [
+    "BaseRunner",
+    "Detectron2GoRunner",
+    "GeneralizedRCNNRunner",
+    "TRAINER_HOOKS_REGISTRY",
+    "create_runner",
+]
 
 
 def create_runner(