Fix backtranslation dataset on IndexedCachedDataset (#410)

Summary:
BacktranslationDataset would throw an error when the underlying dataset was an IndexedCachedDataset because prefetching was not handled correctly. This fixes the error.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/410

Differential Revision: D13557539

Pulled By: myleott

fbshipit-source-id: 398ab59a3ebdbf1c666d862b9f905654eece800c