fix architecture params (#1382)

Summary:
fixes architectures not getting applied to migrated models

Pull Request resolved: fairinternal/fairseq-py#1382

Reviewed By: myleott

Differential Revision: D24603110

Pulled By: alexeib

fbshipit-source-id: 18f44d3736853282466feed5e8896db95338b097

fairseq/models/fairseq_model.py

@@ -40,7 +40,8 @@ def add_args(cls, parser):
         """Add model-specific arguments to the parser."""
         dc = getattr(cls, "__dataclass", None)
         if dc is not None:
-            gen_parser_from_dataclass(parser, dc())
+            # do not set defaults so that settings defaults from various architectures still works
+            gen_parser_from_dataclass(parser, dc(), delete_default=True)
 
     @classmethod
     def build_model(cls, args, task):