message: Change to use assertLen in one test
  due to pylint complaints
diff: @@ -242,7 +242,7 @@ class OptimizerTest(test.TestCase):
  sgd = gradient_descent.SGD(3.0)
  grads_and_vars = sgd._compute_gradients(f, [x])
  - self.assertEqual(1, len(grads_and_vars))
  + self.assertLen(grads_and_vars, 1)
  grad, x_as_var = grads_and_vars[0]
  self.assertIs(x, x_as_var)
  self.assertEqual(2.0, self.evaluate(grad))
  