code: def __len__(self) -> int:
      return len(self.features)
  
imports: from typing import Tuple
  import pytest
  import torch
  from pytest_mock import MockerFixture
  from pytorch_lightning import Trainer
  from torch import Tensor, nn
  from torch.utils.data import DataLoader, Dataset
  from lightly.utils.benchmarking import KNNClassifier
  from lightly.utils.benchmarking.knn_classifier import F
label_desc: Use image node_id 3 for calling the _FeaturesDataset obj's underlying member method code with example usage: obj.__len__() and returns: len