Support for Conv with groups (> 1) != nin

[DrChainsaw]

Current implementation is from the days when DepthwiseConv was the only grouped convolution in Flux. DepthwiseConv was hardcoded to have groups == nin.

I think/hope that constraints can be handled by just applying the current constrains to subsets of the in/out variables. Not sure this will enable the full flexibilty of changing the nin/groups ratio but I'm also not sure if it is meaningful to allow more flexibilty than what the current constraints allow.

What could be the biggest headache is to let the part which creates the new parameter arrays to understand the number of groups and/or the groups/nin ratio from just the selected in/out indicies. Perhaps one needs to be able to pass arbitrary other data in the result from the solver which would require updates to NaiveNASlib.

Will save this one for a rainy day...