Redo: Use `chainer.backward` in gradient_check
Redo: Use `chainer.backward` in gradient_check

This reverts commit fccc53c1670d91d08921e018539ff50272ac8815 and follows the update to `chainer.backward`

okuta/chainer,hvy/chainer,wkentaro/chainer,okuta/chainer,okuta/chainer,niboshi/chainer,okuta/chainer,hvy/chainer,chainer/chainer,hvy/chainer,wkentaro/chainer,wkentaro/chainer,chainer/chainer,hvy/chainer,niboshi/chainer,chainer/chainer,wkentaro/chainer,pfnet/chainer,niboshi/chainer,niboshi/chainer,chainer/chainer