出现非法格式catch住的时候，应该将sample清空

[Gosicfly]

出现非法格式catch住的时候，应该将sample清空，不然会一直报warning，sample也会一直累积新的内容得不到正确的更新。

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 #286
• 添加对var-LSTM的测试代码

Mention: 找人review你的PR

@修改过这个文件的人 @核心开发人员

[codecov-io]

Codecov Report

Merging #264 into master will increase coverage by 0.01%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
+ Coverage   77.42%   77.43%   +0.01%     
==========================================
  Files          84       84              
  Lines       10145    10146       +1     
==========================================
+ Hits         7855     7857       +2     
+ Misses       2290     2289       -1

Impacted Files	Coverage Δ
fastNLP/io/file_reader.py	70.73% <0%> (-0.88%)	:arrow_down:
fastNLP/core/callback.py	68.73% <0%> (+0.35%)	:arrow_up:

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