LanguagePairDataset and BacktranslationDataset changes for semi supervised task setup (#330)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/330

As part of the semi sueprvised task setup (https://github.com/pytorch/translate/pull/243), this diff adds the ability for LanguagePairDataset to remove EOS from source or append EOS to target. This functionality is required by BacktranslationDataset to use translations as source data.

Also added changes to BacktranslationDataset to make it work on GPU. We needed to transfer back-translated sentences back to CPU for the LanguagePairDataset to collate.

Reviewed By: liezl200

Differential Revision: D10846294

fbshipit-source-id: b015ecb5fcef26fba507c30f8a4992bdbc54899f