class_docstr: The partition sampler given a DGLGraph and partition number.
  The metis is used as the graph partition backend.
code: def __getitem__(self, idx):
      return self.par_li[idx]
  
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 3 for calling the ClusterIter obj's underlying member method code with example usage: obj.__getitem__(idx) and returns: self