Fix broken MNIST due to dtype mismatch
Fix broken MNIST due to dtype mismatch

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