Created by: helinwang

Currently CI test do not pass when using WITH_FLUID_ONLY=ON because we have Python fluid tests that depends on paddle.v2 (which is not included when WITH_FLUID_ONLY=ON).

This PR enables us build and test using WITH_FLUID_ONLY flag being set to ON.

• move paddle.v2.dataset,reader to paddle.dataset,reader
• remove unused code (which depends on v2) in paddle.dataset,reader

Fixes: https://github.com/PaddlePaddle/Paddle/issues/8902 , https://github.com/PaddlePaddle/Paddle/issues/9539