class_docstr: The partition sampler given a DGLGraph and partition number.
  The metis is used as the graph partition backend.
code: def __len__(self):
      return self.psize
  
file_path: dgl/examples/pytorch/ogb/cluster-gat/sampler.py
imports: import os
  import torch
  from partition_utils import *
label_desc: Use image node_id 2 for calling the ClusterIter obj's underlying member method code with example usage: obj.__len__() and returns: self