code: def learn(self, obs, label):
      pred_output = self.model(obs)
      cost = (pred_output - obs).pow(2)
      self.optimizer.zero_grad()
      cost.backward()
      self.optimizer.step()
      return cost.item()
  
imports: import numpy
  import unittest
  import os
  import torch
  import torch.nn
  import torch.optim
  import parl
label_desc: Use image node_id 3 for calling the TestAlgorithm obj's underlying member method code with example usage: obj.learn(obs, label) and returns: cost