Cleanup im2col logic
Cleanup im2col logic

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