code: def _construct_label_to_key(labels):
      d = collections.defaultdict(list)
      for i, label in enumerate(labels):
          d[label].append(i)
      return d
  
file_path: chainercv/chainercv/datasets/siamese_dataset.py
imports: import collections
  import numpy
  from chainercv.chainer_experimental.datasets.sliceable import GetterDataset
label_desc: Use image node_id 1 for calling a global function with example usage: _construct_label_to_key(labels) and returns: d