Refactor BacktranslationDataset to be more reusable (#354)

Summary:
- generalize AppendEosDataset -> TransformEosDataset
- remove EOS logic from BacktranslationDataset (use TransformEosDataset instead)
- BacktranslationDataset takes a backtranslation_fn instead of building the SequenceGenerator itself
Pull Request resolved: https://github.com/pytorch/fairseq/pull/354

Reviewed By: liezl200

Differential Revision: D12970233

Pulled By: myleott

fbshipit-source-id: d5c5b0e0a75eca1bd3a50382ac24621f35c32f36