code: def test_helper_tensors():
      """Test the helper tensors."""
      npt.assert_array_equal(qti.e_iso, np.eye(3) / 3)
      npt.assert_array_equal(qti.E_iso, np.eye(6) / 3)
      npt.assert_array_equal(
          qti.E_bulk,
          np.matmul(
              qti.from_3x3_to_6x1(qti.e_iso),
              qti.from_3x3_to_6x1(qti.e_iso).T,
          ),
      )
      npt.assert_array_equal(qti.E_shear, qti.E_iso - qti.E_bulk)
      npt.assert_array_equal(qti.E_tsym, qti.E_bulk + 0.4 * qti.E_shear)
  
imports: import numpy
  import numpy.testing
  import dipy.reconst.qti
  from dipy.core.gradients import gradient_table
  from dipy.sims.voxel import vec2vec_rotmat
  from dipy.core.sphere import disperse_charges, HemiSphere
  from dipy.reconst.dti import fractional_anisotropy
  from dipy.testing.decorators import set_random_number_generator
  from dipy.utils.optpkg import optional_package
label_desc: Use image node_id 7 for calling a global function with example usage: test_helper_tensors() without return types