code: def forward(
      self, msg: Tensor, index: Tensor, t: Tensor, dim_size: int
  ):
      return scatter(
          msg, index, dim=0, dim_size=dim_size, reduce="mean"
      )
  
imports: import copy
  from typing import Callable, Dict, Tuple
  import torch
  from torch import Tensor
  from torch.nn import GRUCell, Linear
  from torch_geometric.nn.inits import zeros
  from torch_geometric.utils import scatter
  from torch_geometric.utils._scatter import scatter_argmax
label_desc: Use image node_id 1 for calling the MeanAggregator obj's underlying member method code with example usage: obj.forward(msg, index, t, dim_size) and returns: scatter