message: Create ModeKeysV2 class which reference TensorFlow ModeKeys constants for consistency. Export ModeKeysV2 as tf.estimator.ModeKeys in V2. Keep existing ModeKeys only exported in V1.
diff: path: "tensorflow.estimator.ModeKeys"
  tf_class {
  - is_instance: "<class \'tensorflow_estimator.python.estimator.model_fn.ModeKeys\'>"
  + is_instance: "<class \'tensorflow_estimator.python.estimator.mode_keys.ModeKeysV2\'>"
  is_instance: "<type \'object\'>"
  member {
  name: "EVAL"
  