Created by: helinwang

https://github.com/PaddlePaddle/Paddle/pull/9414 moved paddle.v2.reader,dataset to paddle.reader,dataset to make fluid no longer depends on v2 (so that compile flag WITH_FLUID_ONLY would work). However, it breaks the backward compatibility.

This PR adds the paddle.v2.reader,dataset back, duplicating with paddle.reader,dataset. I have tried to make paddle.v2.reader,dataset to "reference" paddle.reader,dataset rather than duplicating, but did not work out due to some Python constraints.