feat: Reduce nodes over heterogeneous graphs (#634)

* feat: Reduce nodes over heterogeneous graphs

* test: Add test for reduce

Author: lenianiva

GNNlib/src/utils.jl

@@ -27,6 +27,17 @@ function reduce_nodes(aggr, indicator::AbstractVector, x)
     return NNlib.scatter(aggr, x, indicator)
 end
 
+"""
+    reduce_nodes(aggr, node_type, g, x)
+
+Return the graph-wise aggregation of the node features `x` on type `node_type`
+given a heterogeneous graph `g`. The aggregation operator `aggr` can be `+`,
+`mean`, `max`, or `min`.
+"""
+function reduce_nodes(aggr, node_type, g::GNNHeteroGraph, x)
+    return NNlib.scatter(aggr, x[node_type], graph_indicator(g, node_type))
+end
+
 """
     reduce_edges(aggr, g, e)
 

GNNlib/test/utils.jl

@@ -19,6 +19,17 @@
         @test r2 == r
     end
 
+    @testset "reduce_nodes" begin
+        g = rand_bipartite_heterograph((5, 10), 20)
+        x = (
+            A = [Float32(i) for j = 1:1, i = 1:g.num_nodes[:A]],
+            B = [Float32(0) for j = 1:2, _ = 1:g.num_nodes[:B]],
+        )
+        expected = sum(i for i = 1:g.num_nodes[:A])
+        result = reduce_nodes(+, :A, g, x)
+        @test result == [expected;;]
+    end
+
     @testset "reduce_edges" begin
         r = reduce_edges(mean, g, e)
         @test size(r) == (De, g.num_graphs)