Raise an error if double backprop is performed in forget
Raise an error if double backprop is performed in forget

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