code: def len(self):
      return len(self)
  
imports: import numpy
  import torch
  from torch_geometric.data import Data, Batch
  from torch_points3d.datasets.batch import SimpleBatch
  from torch_points3d.core.data_transform import MultiScaleTransform
  from torch_points3d.datasets.multiscale_data import MultiScaleBatch
  from torch_points3d.datasets.registration.pair import Pair, PairBatch, PairMultiScaleBatch, DensePairBatch
label_desc: Use image node_id 3 for calling the MockDataset obj's underlying member method code with example usage: obj.len() and returns: len