code: def edge_update(self, x_j: Tensor, x_i: Tensor) -> Tensor:
      return x_j - x_i
  
imports: import copy
  from typing import Tuple, Union
  import pytest
  import torch
  from torch import Tensor
  from torch.nn import Linear
  import torch_geometric.typing
  from torch_geometric.nn import MessagePassing, aggr
  from torch_geometric.typing import Adj, OptPairTensor, OptTensor, Size, SparseTensor
  from torch_geometric.utils import add_self_loops, scatter, spmm, to_torch_csc_tensor
label_desc: Use image node_id 3 for calling the MyEdgeConv obj's underlying member method code with example usage: obj.edge_update(x_j, x_i) and returns: unknown