Add test case for set_grad(None)
Add test case for set_grad(None)

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