code: def sparse_lengths_mean_ref(D, I, L):
      return sparse_lengths_sum_ref(D, I, L, normalize_by_lengths=True)
  
imports: from functools import partial
  from hypothesis import given, settings
  import numpy
  import unittest
  import hypothesis.strategies
  from caffe2.python import core, workspace
  import caffe2.python.hypothesis_test_util
  import caffe2.python.serialized_test.serialized_test_util
label_desc: Use image node_id 2 for calling a global function with example usage: sparse_lengths_mean_ref(D, I, L) and returns: sparse_lengths_sum_ref