code: def forward(self, x1, x2, x3):
      x = self.layer1(x1) + self.layer2(x2) + self.layer3(x3)
      return x
  
file_path: hivemind/tests/test_utils/custom_networks.py
imports: import torch
  import torch.nn
  import torch.nn.functional
  from hivemind.moe import register_expert_class
label_desc: Use image node_id 2 for calling the MultiheadNetwork obj's underlying member method code with example usage: obj.forward(x1, x2, x3) and returns: x