Avoid cluttered default args in documentation
Avoid cluttered default args in documentation

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