fix flake8 issue
fix flake8 issue

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