style: Replace None with dy_dx defined variable
style: Replace None with dy_dx defined variable

keisuke-umezawa/chainer,pfnet/chainer,wkentaro/chainer,niboshi/chainer,wkentaro/chainer,hvy/chainer,tkerola/chainer,wkentaro/chainer,okuta/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,okuta/chainer,okuta/chainer,chainer/chainer,keisuke-umezawa/chainer,chainer/chainer,niboshi/chainer,chainer/chainer,wkentaro/chainer,okuta/chainer,niboshi/chainer,hvy/chainer,hvy/chainer,niboshi/chainer,hvy/chainer,chainer/chainer