Use cumprod in backward of prod
Use cumprod in backward of prod

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