Fix transformer LM arg upgrade logic (#1717)

Summary: Pull Request resolved: fairinternal/fairseq-py#1717

Reviewed By: alexeib

Differential Revision: D27156919

Pulled By: myleott

fbshipit-source-id: af3c2e41464c04a7808f40894e7b0106798e4822

fairseq/models/transformer_lm.py

@@ -230,9 +230,6 @@ def __init__(self, decoder):
     def build_model(cls, args, task):
         """Build a new model instance."""
 
-        # make sure all arguments are present in older models
-        base_lm_architecture(args)
-
         if args.decoder_layers_to_keep:
             args.decoder_layers = len(args.decoder_layers_to_keep.split(","))