fix flake8 error
fix flake8 error

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