added check in token block dataset for multiple consecutive blank lines
added check in token block dataset for multiple consecutive blank lines

Summary: Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/830

Differential Revision: D16861799

fbshipit-source-id: d85deaf78ec5b9c23eafd4145a96252e3901fa22

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq