Multilingual training example (#527)

Summary:
* Add example for multilingual translation on IWSLT'17
* Match dataset ordering for multilingual_translation and translation
* Fix bug with LegacyDistributedDataParallel when calling forward of sub-modules
Pull Request resolved: https://github.com/pytorch/fairseq/pull/527

Differential Revision: D14218372

Pulled By: myleott

fbshipit-source-id: 2e3fe24aa39476bcc5c9af68ef9a40192db34a3b