[feature request] suport cuDNN for faster perfomance

[ghost]

I seen some people use cuDNN on waifu2x and they have faster result than without using it. I hope this is useful for waifu2x-chainer

Link:

• https://docs.nvidia.com/deeplearning/sdk/cudnn-install/index.html (for guide)
• http://developer.download.nvidia.com/compute/machine-learning/repos/ (for repo*)

*you don't need an account to install cuDNN, you can install it directly from their repo

[tsurumeso]

I recommend you to install CuPy precompiled binary package that include cuDNN.
Then waifu2x-chainer will use cuDNN automatically.
See: https://docs-cupy.chainer.org/en/stable/install.html#install-cupy

[ghost]

Ohh, automatically. Sorry, I thought I have to do more steps for cuDDN to work.

I use CuPy For CUDA 10.1 if this is something