clean up

transformers/modeling_seq2seq.py

@@ -127,12 +127,8 @@ class PreTrainedSeq2seq(nn.Module):
             decoder = decoder_model
         else:
             kwargs.update(decoder_kwargs)  # Replace encoder kwargs with decoder specific kwargs like config, state_dict, etc...
-            kwargs['is_decoder'] = True  # Make sure the decoder will be an decoder
+            kwargs['is_decoder'] = True  # Make sure the decoder will be a decoder
             decoder = AutoModelWithLMHead.from_pretrained(decoder_pretrained_model_name_or_path, **kwargs)
-            else:
-                raise ValueError("Unrecognized model identifier in {}. Should contains one of "
-                                "'bert', 'openai-gpt', 'gpt2', 'transfo-xl', 'xlnet', "
-                                "'xlm', 'roberta'".format(decoder_pretrained_model_name_or_path))
 
         model = cls(encoder, decoder)
         return model