code: def backward(ctx, grad_output):
      d_output_d_input = ctx.saved_tensors[0]
      lengthscale_grad = grad_output * d_output_d_input
      return None, None, lengthscale_grad, None, None
  
imports: import math
  import torch
label_desc: Use image node_id 2 for calling the MaternCovariance obj's underlying member method code with example usage: obj.backward(ctx, grad_output) and returns: None,  None,  lengthscale_grad,  None,  None