fix scope_session
fix scope_session

xchainer.Device does not hold python backend object (refcount)

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