class_docstr: y = a * x + b
code: def __getitem__(self, index):
      return self.x[index, None], self.y[index, None]
  
imports: from bigdl.orca.learn.pytorch import Estimator
  from unittest import TestCase
  import pytest
  import torch
  import torch.nn
  import os
  import numpy
  from tempfile import TemporaryDirectory
label_desc: Use image node_id 2 for calling the LinearDataset obj's underlying member method code with example usage: obj.__getitem__(index) and returns: self,  self