Fix GroupNorm's arg name caused by external change

Summary: The arg name has been changed by this diff: D7857548

Reviewed By: rbgirshick

Differential Revision: D8283673

fbshipit-source-id: 961dc3773ce7b49d67874323a6dfef738538036d

detectron/modeling/detector.py

@@ -446,7 +446,7 @@ class DetectionModelHelper(cnn.CNNModelHelper):
 
         blob_out = self.SpatialGN(
             conv_blob, prefix + suffix,
-            dim_out, num_groups=group_gn,
+            dim_out, group=group_gn,  # op's arg name is "group"
             epsilon=cfg.GROUP_NORM.EPSILON,)
 
         self.gn_params.append(self.params[-1])  # add gn's bias to list