code: def test_diagonal(self, mat, expected, atol):
      res = Matrices.diagonal(mat)
      self.assertAllClose(res, expected, atol=atol)
  
imports: import pytest
  import geomstats.backend
  from geomstats.geometry.matrices import Matrices
  from geomstats.test.test_case import TestCase
  from geomstats.test.vectorization import generate_vectorization_data
  from geomstats.test_cases.geometry.base import MatrixVectorSpaceTestCase
  from geomstats.test_cases.geometry.euclidean import EuclideanMetricTestCase
label_desc: Use image node_id 16 for calling the MatrixOperationsTestCase obj's underlying member method code with example usage: obj.test_diagonal(mat, expected, atol) without return types