code: def backward(ctx, grad_output):
      raise NotImplementedError("GMM does not support backpropagation")
  
file_path: MONAI/monai/networks/layers/gmm.py
imports: from __future__ import annotations
  import torch
  from monai._extensions.loader import load_module
label_desc: Use image node_id 2 for calling the _ApplyFunc obj's underlying member method code with example usage: obj.backward(ctx, grad_output) without return types