Modify eval_bleu_args to Optional (#3175)
Modify eval_bleu_args to Optional (#3175)

Summary:
# Before submitting

- [x] 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)?
- [ ] Did you make sure to update the docs?
- [ ] Did you write any new necessary tests?

## What does this PR do?
Fixes https://github.com/pytorch/fairseq/issues/3158 .

## PR review
Fixes bug of loading previously trained translation-task model. It is not necessary to define eval_bleu_args and eval_bleu_detok_args when we use eval_bleu=False.

## Did you have fun?
Make sure you had fun coding �

Pull Request resolved: https://github.com/pytorch/fairseq/pull/3175

Reviewed By: alexeib

Differential Revision: D26225882

Pulled By: myleott

fbshipit-source-id: ec5908179560cc44c31bac29831beb62dd81305d

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq