code: def __len__(self):
      return self.len
  
file_path: nni/test/ut/nas/experiment/test_experiment.py
imports: import torch
  from torch import nn
  from torch.optim import SGD
  from torch.utils.data import DataLoader, Dataset
  import nni
  from nni.nas.evaluator import FunctionalEvaluator
  from nni.nas.evaluator.pytorch import Classification
  from nni.nas.experiment import *
  from nni.nas.experiment.config import *
  from nni.nas.strategy import RegularizedEvolution, PolicyBasedRL, DARTS, Random
  from ut.nas.nn.models import SimpleNet
label_desc: Use image node_id 3 for calling the RandomMnistDataset obj's underlying member method code with example usage: obj.__len__() and returns: self