filter no used layers, when loading datasets minus mean value as default

[liuhuiCNN]

1.加载预训练模型时，如果自己的数据集分类数不是1000类，需要过滤不需要的layer 2.加载数据时默认减去mean值

• 如果加载预训练模型，需要将mean值设置跟预训练模型一致
• 如果不加载预训练模型，加载数据也要考虑归一化的问题

[qingqing01]

https://travis-ci.org/PaddlePaddle/models/builds/365580859?utm_source=github_status&utm_medium=notification

代码格式不对，需要安装pre-commit，请参考：

http://www.paddlepaddle.org/docs/develop/documentation/zh/dev/contribute_to_paddle_cn.html#pre-commit

[CLAassistant]

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