How to use GPU(s) for acceleration when testing?

Dear author(s),

Thanks for your interesting work.

I am now using other data to test the effect of this method, but I found that the test time is too long (mainly forward propagation and backward propagation), and then I found that the GPU is not run while testing, so I would like to ask how to use GPU(s) to accelerate when testing? 

Thank you!

Gavin