AggregationLayer
AggregationLayer[f] represents a layer that aggregates an array of arbitrary rank into a vector, using the function f.
AggregationLayer[f,n] aggregates an array at level n.
AggregationLayer[f,n1;;n2] aggregates an array at levels n1 through n2.
AggregationLayer[f,{n1,n2,…}] aggregates an array at levels n1,n2,….
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