Add cat_union

[jbjeong]

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Hello,

I made a simple function to concatenate the tensors which have different sparsity. This can be used in UNet based auto-encoder, where generated tensors and skipped tensors have different sparsity.

Thanks

[chrischoy]

Thanks for your contribution.

However, this function requires creating two existing coords map A.C, B.C. Could you use get_union_map on https://github.com/StanfordVL/MinkowskiEngine/blob/master/MinkowskiEngine/MinkowskiCoords.py#L314 to remove the unnecessary coordinate creation?