Fix iteration bug in GroupedIterator. Correct sent size filter. (#455)

Summary:
Fix iterating from the beginning bug when initializing the GroupedIterator. (https://github.com/pytorch/fairseq/issues/441)
 Correct filter criterion for dict type sentence size. (https://github.com/pytorch/fairseq/issues/451)
Pull Request resolved: https://github.com/pytorch/fairseq/pull/455

Differential Revision: D13725646

Pulled By: myleott

fbshipit-source-id: e698fa6f9b45460f95a75c9e9976a3aa3b6aa523