class_docstr: y = a * x + b
code: def __len__(self):
      return len(self.x)
  
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 3 for calling the LinearDataset obj's underlying member method code with example usage: obj.__len__() and returns: len