Add size method to BacktranslationDataset + misc fixes (#325)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/325

RoundRobinZipDataset requires size(index) method implemented in every dataset used. Also added missing return statements in a few methods.

Reviewed By: liezl200

Differential Revision: D10457159

fbshipit-source-id: 01856eb455f2f3a21e7fb723129ff35fbe29e0ae