Please feel free to remove inapplicable items for your PR.
[y] The PR title starts with [$CATEGORY] (例如[bugfix]修复bug，[new]添加新功能，[test]修改测试，[rm]删除旧代码)
[y] Changes are complete (i.e. I finished coding on this PR) 修改完成才提PR
[y] All changes have test coverage 修改的部分顺利通过测试。对于fastnlp/fastnlp/的修改，测试代码必须提供在fastnlp/test/。
[y] Code is well-documented 注释写好，API文档会从注释中抽取
[y] To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change 修改导致例子或tutorial有变化，请找核心开发人员
Description：增加了困惑度指标。
Main reason: 增加了困惑度指标，并且对于困惑度进行了测试。
Checklist 检查下面各项是否完成
Please feel free to remove inapplicable items for your PR.
Changes: 逐项描述修改的内容
@修改过这个文件的人 @核心开发人员