Refactor TensorNetwork.backward_cpu and TensorNetwork.backward_gpu
Refactor TensorNetwork.backward_cpu and TensorNetwork.backward_gpu

cupy/cupy,woodshop/complex-chainer,keisuke-umezawa/chainer,1986ks/chainer,umitanuki/chainer,wkentaro/chainer,laysakura/chainer,chainer/chainer,anaruse/chainer,ytoyama/yans_chainer_hackathon,hvy/chainer,sou81821/chainer,cupy/cupy,pfnet/chainer,benob/chainer,masia02/chainer,truongdq/chainer,kikusu/chainer,okuta/chainer,tkerola/chainer,ronekko/chainer,wkentaro/chainer,cemoody/chainer,yanweifu/chainer,tscohen/chainer,AlpacaDB/chainer,kuwa32/chainer,t-abe/chainer,muupan/chainer,cupy/cupy,truongdq/chainer,niboshi/chainer,ysekky/chainer,hvy/chainer,jnishi/chainer,tigerneil/chainer,sinhrks/chainer,okuta/chainer,woodshop/chainer,Kaisuke5/chainer,chainer/chainer,muupan/chainer,niboshi/chainer,t-abe/chainer,chainer/chainer,jnishi/chainer,niboshi/chainer,kikusu/chainer,jnishi/chainer,wkentaro/chainer,niboshi/chainer,keisuke-umezawa/chainer,hidenori-t/chainer,ktnyt/chainer,ikasumi/chainer,kashif/chainer,ktnyt/chainer,aonotas/chainer,jnishi/chainer,minhpqn/chainer,sinhrks/chainer,okuta/chainer,ktnyt/chainer,cupy/cupy,hvy/chainer,benob/chainer,wkentaro/chainer,wavelets/chainer,AlpacaDB/chainer,kiyukuta/chainer,chainer/chainer,okuta/chainer,hvy/chainer,rezoo/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,delta2323/chainer,ktnyt/chainer