Open Source MLM Implementation in Fairseq (#635)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/635

Adding a task and relevant models, datasets and criteria needed for training Cross-lingual Language Models similar to Masked Language Model used in XLM (Lample and Conneau, 2019 - https://arxiv.org/abs/1901.07291).

Reviewed By: liezl200

Differential Revision: D14943776

fbshipit-source-id: 3e416a730303d1dd4f5b92550c78db989be27073