LMDB格式数据配置和模型训练配置，适用大数据模型训练数据配置

[huiyang865]

在mmclassification v0.6.0基础上添加代码如下：

1. 持久化 pickle和lmdb格式代码；
2. 添加MyDataset数据类，包括直接读取txt文档数据和LMDB数据库两种形式；
3. 添加lmdb数据库训练模型的config文件。

结果：亲测可以成功持久化LMDB数据，并训练模型。适合用户大数据量模型训练的配置。

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[huiyang865]

More details of this request refer to #79

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@huiyang865
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