FIX: bos is always at 0th element (#2369)
FIX: bos is always at 0th element (#2369)

Summary:
small bug fix in dataset creation

Pull Request resolved: https://github.com/pytorch/fairseq/pull/2369

Reviewed By: pipibjc

Differential Revision: D22727599

Pulled By: myleott

fbshipit-source-id: bb7f18d85b72a19667e2a6844bbe172a3397bafb

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq