Improve extensions part in trainer tutorial
Improve extensions part in trainer tutorial

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