Update hub_utils.py (#2910)

Summary:
# Before submitting

- [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
- [x] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/master/CONTRIBUTING.md)?
- [x] Did you make sure to update the docs?
- [ ] Did you write any new necessary tests?

## What does this PR do?

 has no attribute 'arg'

Pull Request resolved: #2910

Reviewed By: alexeib

Differential Revision: D25146481

Pulled By: myleott

fbshipit-source-id: 11912bb2bcacd1d2f91da47bb0d868da90b38f17

fairseq/hub_utils.py

@@ -180,7 +180,7 @@ def generate(
         if verbose:
 
             def getarg(name, default):
-                return getattr(gen_args, name, getattr(self.args, name, default))
+                return getattr(gen_args, name, getattr(self.cfg, name, default))
 
             for source_tokens, target_hypotheses in zip(tokenized_sentences, outputs):
                 src_str_with_unk = self.string(source_tokens)