Add XGLM downstream task evaluation examples (#4154)
Add XGLM downstream task evaluation examples (#4154)

Summary:
1. Add XGLM downstream task evaluation examples
2. Add bibtex citation of XGLM arXiv paper

Pull Request resolved: https://github.com/pytorch/fairseq/pull/4154

Reviewed By: xianxl

Differential Revision: D33748846

Pulled By: todpole3

fbshipit-source-id: ce4dfce2fccf92742f124f12a0d9a388280320fa

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq