SpatialConvolutionMM now support on-the-fly zero-padding.

Interface similar to cunn.SpatialConvolutionMM.

To get a layer where osize == isize, do this:
  local ksize = 5
  local padding = (ksize-1)/2
  local n = nn.SpatialConvolutionMM(ni,no, ksize,ksize, 1,1, padding)

Note: dW/dH still not supported on the CPU version (strides).