add cn doc for GPU DataLoader

[heavengate]

add cn doc for GPU DataLoader

[paddle-bot-old[bot]]

Thanks for your contribution!

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}