add missing override
add missing override

Co-Authored-By: Hiroyuki Vincent Yamazaki <a5848102d8491b6959f66fb5d5231fc8bd6a56a0@gmail.com>
wkentaro/chainer,okuta/chainer,hvy/chainer,niboshi/chainer,chainer/chainer,tkerola/chainer,hvy/chainer,wkentaro/chainer,okuta/chainer,wkentaro/chainer,keisuke-umezawa/chainer,chainer/chainer,wkentaro/chainer,pfnet/chainer,keisuke-umezawa/chainer,niboshi/chainer,chainer/chainer,chainer/chainer,niboshi/chainer,keisuke-umezawa/chainer,niboshi/chainer,okuta/chainer,hvy/chainer,hvy/chainer,keisuke-umezawa/chainer,okuta/chainer