message: Use ModeKeys constants in Model._make_execution_function.
diff: @@ -48,6 +48,7 @@ from tensorflow.python.ops import math_ops
  from tensorflow.python.platform import tf_logging as logging
  from tensorflow.python.training import optimizer as tf_optimizer_module
  from tensorflow.python.training.checkpointable import base as checkpointable
  +from tensorflow.python.training.mode_keys import ModeKeys
  from tensorflow.python.util import nest
  from tensorflow.python.util.tf_export import tf_export
  @@ -2019,13 +2020,13 @@ class Model(Network):
  **kwargs)
  def _make_execution_function(self, mode):
  - if mode == 'train':
  + if mode == ModeKeys.TRAIN:
  self._make_fit_function()
  return self._fit_function
  - if mode == 'test':
  + if mode == ModeKeys.TEST:
  self._make_eval_function()
  return self._eval_function
  - if mode == 'predict':
  + if mode == ModeKeys.PREDICT:
  self._make_predict_function()
  return self.predict_function
  