Fix backward implementation
Fix backward implementation

Co-Authored-By: Takeshi Watanabe <437db7eeec5e403f3355810727c8f1ab67683110@users.noreply.github.com>
niboshi/chainer,niboshi/chainer,wkentaro/chainer,hvy/chainer,wkentaro/chainer,chainer/chainer,hvy/chainer,pfnet/chainer,niboshi/chainer,hvy/chainer,wkentaro/chainer,hvy/chainer,chainer/chainer,chainer/chainer,chainer/chainer,niboshi/chainer,wkentaro/chainer