code: def __len__(self):
      return len(self.x)
  
imports: from unittest import TestCase
  from bigdl.orca.automl.model.base_pytorch_model import PytorchModelBuilder
  from torch.utils.data import Dataset, DataLoader
  import pytest
  import numpy
  import torch
  import torch.nn
label_desc: Use image node_id 2 for calling the LinearDataset obj's underlying member method code with example usage: obj.__len__() and returns: len