Update chainer/training/extensions/variable_unchain.py
Update chainer/training/extensions/variable_unchain.py

Co-Authored-By: hitsgub <e131c83cac67b8b9c0b08c9a453f1229fb04ecc9@gmail.com>
niboshi/chainer,keisuke-umezawa/chainer,wkentaro/chainer,chainer/chainer,okuta/chainer,chainer/chainer,okuta/chainer,okuta/chainer,keisuke-umezawa/chainer,wkentaro/chainer,keisuke-umezawa/chainer,wkentaro/chainer,chainer/chainer,tkerola/chainer,okuta/chainer,hvy/chainer,hvy/chainer,keisuke-umezawa/chainer,wkentaro/chainer,niboshi/chainer,niboshi/chainer,hvy/chainer,hvy/chainer,chainer/chainer,pfnet/chainer,niboshi/chainer