Group normalization with sparse convolution

[victoryc]

I see that SparseConvNet has implementation for batch normalization. Is it possible to use group normalization with SparseConvNet?

My specific use case is the following. As of now, I am using the U-Net class available in SparseConvNet. But, it uses batch normalization. I would like try out group normalization instead.

Thanks.

[btgraham]

Added as sparseconvnet.SparseGroupNorm