Have noising account for sentences with and without EOS (#305)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/305

Previously, noising code assumed that every sentence had an EOS which had to be excluded from noising operations (since we shouldn't drop, blank, or shuffle EOS). This logic allows the noising module to handle sentences with EOS and without EOS

Reviewed By: xianxl

Differential Revision: D10114425

fbshipit-source-id: 04ec8547343eb94266bda1ac7fca3d8a1991c9f4