class_docstr: Collect pair-wise graph data to calculate marginal contribution. 
code: def __len__(self):
      return self.exclude_mask.shape[0]
  
imports: import torch
  from torch_geometric.data import Data, Dataset
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