message: Fix control flow test to not use session after it's gone out of scope.
  This somehow works currently, but breaks with the C API enabled.
diff: @@ -1450,7 +1450,8 @@ class ControlFlowTest(test.TestCase):
  gpu_dev_name = test.gpu_device_name() if test.is_gpu_available(
  ) else "/device:GPU:0"
  - with self.test_session(graph=ops.Graph()) as sess:
  + graph = ops.Graph()
  + with graph.as_default():
  v = constant_op.constant(2.0, name="v")
  c = lambda v: math_ops.less(v, 100.0)
  @@ -1461,7 +1462,8 @@ class ControlFlowTest(test.TestCase):
  loop = control_flow_ops.while_loop(c, b, [v], parallel_iterations=1)
  r = gradients_impl.gradients(
  loop, v, colocate_gradients_with_ops=colocate)[0]
  - r_ops = r.graph.get_operations()
  +
  + r_ops = graph.get_operations()
  r_devices = [(op.name, op.device) for op in r_ops]
  self.assertTrue(any("Square" in op.name for op in r_ops))
  @@ -1475,6 +1477,8 @@ class ControlFlowTest(test.TestCase):
  self.assertTrue(gpu_dev_name in dev)
  else:
  self.assertFalse(gpu_dev_name in dev)
  +
  + with self.test_session(graph=graph) as sess:
  self.assertAllClose(1024.0, sess.run(r))
  def testWhileGrad_ColocateGradients(self):
  