code: def test_gcn_conv_flow():
      x = torch.randn(4, 16)
      edge_index = torch.tensor([[0, 0, 0], [1, 2, 3]])
  
      conv = GCNConv(16, 32, flow="source_to_target")
      out1 = conv(x, edge_index)
      conv.flow = "target_to_source"
      out2 = conv(x, edge_index.flip(0))
      assert torch.allclose(out1, out2, atol=1e-6)
  
file_path: pytorch_geometric/test/nn/conv/test_gcn_conv.py
imports: import pytest
  import torch
  import torch_geometric.typing
  from torch_geometric.nn import GCNConv
  from torch_geometric.nn.conv.gcn_conv import gcn_norm
  from torch_geometric.testing import is_full_test
  from torch_geometric.typing import WITH_PT21, SparseTensor
  from torch_geometric.utils import to_torch_coo_tensor, to_torch_csc_tensor
label_desc: Use image node_id 6 for calling a global function with example usage: test_gcn_conv_flow() without return types