Small Hydra fix (facebookresearch#1543)

Summary: Pull Request resolved: fairinternal/fairseq-py#1543

Test Plan: Imported from OSS

Reviewed By: girifb

Differential Revision: D25836852

Pulled By: myleott

fbshipit-source-id: 7fda711d21f2d1b7bac26792233997e8dea2f835

fairseq/dataclass/utils.py

@@ -162,7 +162,7 @@ def get_kwargs_from_dc(
                     continue
                 else:
                     del kwargs["default"]
-            if delete_default:
+            if delete_default and "default" in kwargs:
                 del kwargs["default"]
         try:
             parser.add_argument(*field_args, **kwargs)