computing error for IM2COL with kernel size=3

WeixiangXu

``` src = Variable(torch.randn(8,7,7).cuda()) k = 3 pad = 0 s = (1,1) dst = P.im2col(src, k, s, pad) back = P.col2im(dst, k, s, pad) self.assertEqual((src - back).data.abs().max(), 0) ```