Refactoring: Use the inbuilt im2col NHWC?

More of a code cleanliness thing, but currently for im2col in nhwc layout we use our own method that I took from the pytorch docs. ONNXRuntime also includes its own im2col nhwc implementation (see the cpu math ops). The two functions are basically identical except in our own im2col_nhwc function it does it for all groups whereas the ORT one only does it for one group. So when you refactor you'll have to move the im2col inside the for-loop where we iterate over the groups.