Fix DiscreteDqnInput.from_dict (facebookresearch#506)

Summary:
Pull Request resolved: facebookresearch#506

Use `ExtraData.from_dict`

Reviewed By: czxttkl

Differential Revision: D29768249

fbshipit-source-id: de0056420ab71a79c4f9821cf451328949256037

reagent/core/types.py

@@ -757,7 +757,7 @@ def from_dict(cls, batch):
             next_action=batch[InputColumn.NEXT_ACTION],
             possible_actions_mask=batch[InputColumn.POSSIBLE_ACTIONS_MASK],
             possible_next_actions_mask=batch[InputColumn.POSSIBLE_NEXT_ACTIONS_MASK],
-            extras=batch[InputColumn.EXTRAS],
+            extras=ExtraData.from_dict(batch),
         )