code: def test_multi_aggregation_resolver():
      aggr = aggregation_resolver(None)
      assert aggr is None
  
      aggr = aggregation_resolver(["sum", "mean", None])
      assert isinstance(aggr, torch_geometric.nn.MultiAggregation)
      assert len(aggr.aggrs) == 3
      assert isinstance(
          aggr.aggrs[0], torch_geometric.nn.SumAggregation
      )
      assert isinstance(
          aggr.aggrs[1], torch_geometric.nn.MeanAggregation
      )
      assert aggr.aggrs[2] is None
  
file_path: pytorch_geometric/test/nn/test_resolver.py
imports: import pytest
  import torch
  from torch.optim.lr_scheduler import ConstantLR, LambdaLR, ReduceLROnPlateau
  import torch_geometric
  from torch_geometric.nn.resolver import activation_resolver, aggregation_resolver, lr_scheduler_resolver, normalization_resolver, optimizer_resolver
label_desc: Use image node_id 3 for calling a global function with example usage: test_multi_aggregation_resolver() without return types