Back translation + denoising in MultilingualTranslation task (#620)

Summary:
- Add language token to MultilingualTranslation task
- Add back translation and denoising loss to MultilingualTranslation task
Pull Request resolved: https://github.com/pytorch/fairseq/pull/620

Reviewed By: liezl200

Differential Revision: D14756873

Pulled By: pipibjc

fbshipit-source-id: 89d668db26848fd95f446edf5923bab2113636f7