code: def __len__(self):
      return self.node_state.shape[0]
  
imports: import copy
  import os
  import dgl
  import networkx
  import numpy
  import torch
  from torch.utils.data import DataLoader, Dataset
label_desc: Use image node_id 2 for calling the MultiBodyDataset obj's underlying member method code with example usage: obj.__len__() and returns: self