[verify] train_char_cnn optimization

[SrWYG]

Description：简要描述这次PR的内容

Main reason: 做出这次修改的原因

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 [$CATEGORY] (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/。
• Code is well-documented 注释写好，API文档会从注释中抽取
• 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
• 修改dataset.py中过时的和不合规则的注释 将pytorch模型转换为fastNLP可用的模型 #286
• 添加对var-LSTM的测试代码

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