Improve _check_grad_type
Improve _check_grad_type

jnishi/chainer,keisuke-umezawa/chainer,aonotas/chainer,okuta/chainer,jnishi/chainer,wkentaro/chainer,tkerola/chainer,rezoo/chainer,wkentaro/chainer,ktnyt/chainer,niboshi/chainer,hvy/chainer,pfnet/chainer,wkentaro/chainer,ktnyt/chainer,chainer/chainer,keisuke-umezawa/chainer,chainer/chainer,ktnyt/chainer,okuta/chainer,wkentaro/chainer,ronekko/chainer,ktnyt/chainer,niboshi/chainer,chainer/chainer,keisuke-umezawa/chainer,niboshi/chainer,hvy/chainer,hvy/chainer,okuta/chainer,jnishi/chainer,jnishi/chainer,niboshi/chainer,anaruse/chainer,chainer/chainer,hvy/chainer,keisuke-umezawa/chainer,okuta/chainer,kashif/chainer