code: def backward(ctx, grad_output):
      return grad_output.neg()
  
file_path: pytext/pytext/utils/loss.py
imports: import numpy
  import torch
  from .cuda import FloatTensor
label_desc: Use image node_id 2 for calling the LagrangeMultiplier obj's underlying member method code with example usage: obj.backward(ctx, grad_output) and returns: grad_output