code: def __len__(self):
      return self.exclude_mask.shape[0]
  
imports: import copy
  import torch
  import numpy
  from scipy.special import comb
  from itertools import combinations
  import torch.nn.functional
  from torch_geometric.utils import to_networkx
  from torch_geometric.data import Data, Batch, Dataset
  from torch_geometric.loader import DataLoader
label_desc: Use image node_id 2 for calling the MarginalSubgraphDataset obj's underlying member method code with example usage: obj.__len__() and returns: self