Upgrade args: max_sentences to batch_size (#2754)
Upgrade args: max_sentences to batch_size (#2754)

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

## What does this PR do?

Upgrade args: `max_sentences` to `batch_size`

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

Pull Request resolved: https://github.com/pytorch/fairseq/pull/2754

Reviewed By: alexeib

Differential Revision: D24418980

Pulled By: myleott

fbshipit-source-id: 5269c2fc8c434513cc5114f7e9d2eccd0c553fbd

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq