message: Deprecate `set_learning_phase()` backend utility.
  Deprecating the getter (`learning_phase()`) can't be done at this time because it is used in all core layers. Changing it in the core layers requires changing the default value of the `training` arg in their call methods, which is not a backwards compatible change.
diff: @@ -393,6 +393,9 @@ def _default_learning_phase():
  False, shape=(), name='keras_learning_phase')
  +@deprecated('2020-10-11',
  + 'Simply pass a True/False value to the `training` argument '
  + 'of the `__call__` method of your layer or model.')
  @keras_export('keras.backend.set_learning_phase')
  def set_learning_phase(value):
  """Sets the learning phase to a fixed value.
  