Add type hints for ops.Graph in control_flow_util_test.py.

PiperOrigin-RevId: 557537614

Author: tensorflower-gardener

tensorflow/python/kernel_tests/control_flow/control_flow_util_test.py

@@ -72,7 +72,7 @@ def testIsLoopExit(self):
 
     self.assertFalse(control_flow_util.IsLoopExit(test_ops.int_output().op))
 
-  def build_test_graph(self):
+  def build_test_graph(self) -> ops.Graph:
     g = ops.Graph()
     with g.as_default():