use "legacy" dataclass at operator level

Summary: Pull Request resolved: #444

Differential Revision: D41828774

fbshipit-source-id: 3ce2114db4da61b4343d5c1a52d9c44a46f59671

Author: Yanghan Wang

d2go/trainer/api.py

@@ -18,7 +18,7 @@ class TrainNetOutput:
     metrics: MetricsDict[float]
     # Optional, because we use None to distinguish "not used" from
     # empty model configs. With T127368935, this should be reverted to dict.
-    model_configs: Optional[Dict[str, str]]
+    model_configs: Dict[str, str]
     # TODO (T127368603): decide if `tensorboard_log_dir` should be part of output
     tensorboard_log_dir: Optional[str] = None