GroupNormalization is as important as LayerNormalization and BatchNormalization. The current NormalizationLayer doesn't have an option for normalizing across groups of channels. It can be implemented by a sequence of reshaping and normalization layers.

GroupNormalization[nGroups_, inputShape_] := 
   NetChain[{ 
(*assuming the channel dimension is the first one*)
    ReshapeLayer[{nGroups, inputShape[[1]]/nGroups, Inherited, 
      Automatic}], NormalizationLayer[2 ;;, "Complement"], 
    ReshapeLayer[inputShape]}, "Input" -> inputShape]

It would be great if the number of groups option was added to LayerNormalization or create a new layer for only this purpose.