non-deterministic result when using cuDNN and upconv_7

[nagadomi]

This issue is related to: lltcggie/waifu2x-caffe#41

Sometimes, upscaled image by waifu2x is unreproducible.
upconv_7 (default upscaling model of latest waifu2x) has Deconvolution layer(aka FullConvolution,Upconvolution). Deconvolution layer on cudnn.torch is implemented by the backward method of Convolution layer (swapping Forward/Backward method). And, backward method of Convolution layer on cuDNN is non-deterministic method. So the forward path of upconv_7 is non-deterministic.
The pixel-wise differences of result images that are caused by this issue is a very little. I think this issue is a small matter. but I was not intended this behavior.