code: def _retrieve_mnist_training():
      urls = [
          "http://yann.lecun.com/exdb/mnist/train-images-idx3-ubyte.gz",
          "http://yann.lecun.com/exdb/mnist/train-labels-idx1-ubyte.gz",
      ]
      return _retrieve_mnist("train.npz", urls)
  
file_path: chainer/chainer/datasets/mnist.py
imports: import os
  import numpy
  import chainer
  from chainer.dataset import download
  from chainer.datasets._mnist_helper import make_npz
  from chainer.datasets._mnist_helper import preprocess_mnist
label_desc: Use image node_id 2 for calling a global function with example usage: _retrieve_mnist_training() and returns: _retrieve_mnist