code: def test_combine_mean_std():
      """Test invoking CombineMeanStd."""
      mean = np.random.rand(5, 3).astype(np.float32)
      std = np.random.rand(5, 3).astype(np.float32)
      layer = layers.CombineMeanStd(
          training_only=True, noise_epsilon=0.01
      )
      result1 = layer([mean, std], training=False)
      assert np.array_equal(result1, mean)  # No noise in test mode
      result2 = layer([mean, std], training=True)
      assert not np.array_equal(result2, mean)
      assert np.allclose(result2, mean, atol=0.1)
  
imports: import deepchem
  import numpy
  import pytest
  import os
label_desc: Use image node_id 3 for calling a global function with example usage: test_combine_mean_std() without return types