Add forward declaration to solve dependency
Add forward declaration to solve dependency

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