Description：修复issue322中提到的bug ； 增加requirement文件；
Main reason: bug修复
Checklist 检查下面各项是否完成
Please feel free to remove inapplicable items for your PR.
[x] 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)
[x] Changes are complete (i.e. I finished coding on this PR) 修改完成才提PR
[x] 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/。
[x] Code is well-documented 注释写好，API文档会从注释中抽取
[x] 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：修复issue322中提到的bug ； 增加requirement文件； Main reason: bug修复
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
Mention: 找人review你的PR
@yhcc