code: def message(self, x_j, y_j, z_j):
      return self.lin(torch.cat([x_j, y_j, z_j], dim=-1))
  
imports: import random
  import pytest
  import torch
  from torch_geometric.data import HeteroData
  from torch_geometric.datasets import FakeHeteroDataset
  from torch_geometric.nn import GATConv, GCN2Conv, GCNConv, HeteroConv, Linear, MessagePassing, SAGEConv
  from torch_geometric.profile import benchmark
  from torch_geometric.testing import get_random_edge_index, onlyLinux, withCUDA, withPackage
label_desc: Use image node_id 3 for calling the CustomConv obj's underlying member method code with example usage: obj.message(x_j, y_j, z_j) and returns: self