message: Checkpointable: remove colocation constraints from restore ops
  Mystery solved thanks to log_device_placement.
diff: @@ -764,7 +764,6 @@ class CheckpointingTests(test.TestCase):
  checkpoint_directory = self.get_temp_dir()
  root = checkpointable.Checkpointable()
  - with ops.device("/cpu:0"):
  root.var = checkpointable_utils.add_variable(
  root, name="var", initializer=0.)
  optimizer = adam.AdamOptimizer(0.1)
  @@ -796,7 +795,6 @@ class CheckpointingTests(test.TestCase):
  new_root).restore(no_slots_path)
  with self.assertRaises(AssertionError):
  no_slot_status.assert_consumed()
  - with ops.device("/cpu:0"):
  new_root.var = checkpointable_utils.add_variable(
  new_root, name="var", shape=[])
  no_slot_status.assert_consumed()
  