Revert changes in optimizer.py
Revert changes in optimizer.py

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