Custom Reader

[JiayiFeng]

In our design, reader is used to read data from files as well as doing data preprocessing. A few basic and common data preprocessing has been implemented by us(batching, shuffling,...) in the form of DecoratedReader. However, there are unlimited ways to do data preprocessing, so it is necessary to introduce a 'custom reader', with which users are able to config their own preprocessing pipelines.

CustomReader is also a DecoartedReader. Unlike other decorated readers, a CustomReader holds its own sub-block. Users can config their own preprocessing pipelines in the sub-block, and the sub-block will be executed in the CustomReader's ReadNext() function.