Main reason: 使用MLP([], activation=lambda x: x)出现错误
Checklist 检查下面各项是否完成
Please feel free to remove inapplicable items for your PR.
[ ] The PR title starts with [$bugfix] (例如[bugfix]修复bug，[new]添加新功能，[test]修改测试，[rm]删除旧代码)
[ ] Changes are complete (i.e. I finished coding on this PR) 修改完成才提PR
[ ] 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/。
[ ] 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
Changes: 逐项描述修改的内容
添加了新模型；用于句子分类的CNN，来自Yoon Kim的Convolutional Neural Networks for Sentence Classification
Description：MLP的小bug
Main reason: 使用MLP([], activation=lambda x: x)出现错误
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
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@修改过这个文件的人 @核心开发人员