make IntervalTrigger stateful to fix PTB example bug
make IntervalTrigger stateful to fix PTB example bug

kashif/chainer,delta2323/chainer,okuta/chainer,cupy/cupy,chainer/chainer,jnishi/chainer,rezoo/chainer,chainer/chainer,ktnyt/chainer,keisuke-umezawa/chainer,hvy/chainer,pfnet/chainer,ktnyt/chainer,aonotas/chainer,anaruse/chainer,jnishi/chainer,hvy/chainer,hvy/chainer,okuta/chainer,cupy/cupy,ronekko/chainer,keisuke-umezawa/chainer,hvy/chainer,niboshi/chainer,keisuke-umezawa/chainer,niboshi/chainer,cupy/cupy,jnishi/chainer,keisuke-umezawa/chainer,okuta/chainer,chainer/chainer,ysekky/chainer,jnishi/chainer,okuta/chainer,wkentaro/chainer,ktnyt/chainer,niboshi/chainer,tkerola/chainer,cupy/cupy,niboshi/chainer,chainer/chainer,wkentaro/chainer,kiyukuta/chainer,ktnyt/chainer,wkentaro/chainer,wkentaro/chainer