Fix generation with --no-early-stop (#627)

Summary:
Because the size of `unfinalized_scores` is equal to current `bsz` and not initial batch size, we need to index it by `unfin_idx` instead of `sent` in `is_finished`.
Fixes #588.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/627

Differential Revision: D15034641

Pulled By: myleott

fbshipit-source-id: 2638e68e877ae01256cac7d8e69b5b7fec8f7017