Support ChainerX in PTB gentxt example
Support ChainerX in PTB gentxt example

keisuke-umezawa/chainer,pfnet/chainer,hvy/chainer,chainer/chainer,chainer/chainer,niboshi/chainer,niboshi/chainer,okuta/chainer,wkentaro/chainer,okuta/chainer,keisuke-umezawa/chainer,niboshi/chainer,hvy/chainer,hvy/chainer,okuta/chainer,wkentaro/chainer,chainer/chainer,keisuke-umezawa/chainer,wkentaro/chainer,chainer/chainer,keisuke-umezawa/chainer,hvy/chainer,niboshi/chainer,okuta/chainer,wkentaro/chainer