GitLab

Summary:
This task and loss are used for sentence ranking and multiple choice tasks such as RACE
Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/809

Reviewed By: myleott

Differential Revision: D16715745

Pulled By: jingfeidu

fbshipit-source-id: cb4d1c7b26ebb3e2382449ba51af5745ef56f30f