Update language_pair_dataset.py (#2367)
Update language_pair_dataset.py (#2367)

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?
- [x] Did you write any new necessary tests?

## What does this PR do?

add sort_order for `prev_output_tokens`

Pull Request resolved: https://github.com/pytorch/fairseq/pull/2367

Reviewed By: pipibjc

Differential Revision: D22727492

Pulled By: myleott

fbshipit-source-id: 932f073b3b938682d2189e6c072a26bee7169a98

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq