message: Add doctest for tf.keras.backend.clear_session
diff: @@ -257,9 +257,29 @@ def reset_uids():
  @keras_export('keras.backend.clear_session')
  def clear_session():
  - """Destroys the current TF graph and creates a new one.
  -
  - Useful to avoid clutter from old models / layers.
  + """Destroys the current TF graph and session, and creates a new one.
  +
  + Calling clear_session() releases the global graph state that Keras is
  + holding on to; resets the counters used for naming layers and
  + variables in Keras; and resets the learning phase. This helps avoid clutter
  + from old models and layers, especially when memory is limited, and a
  + common use-case for clear_session is releasing memory when building models
  + and layers in a loop.
  +
  + >>> import tensorflow as tf
  + >>> layers = [tf.keras.layers.Dense(10) for _ in range(10)]
  + >>> new_layer = tf.keras.layers.Dense(10)
  + >>> print(new_layer.name)
  + dense_10
  + >>> tf.keras.backend.set_learning_phase(1)
  + >>> print(tf.keras.backend.learning_phase())
  + 1
  + >>> tf.keras.backend.clear_session()
  + >>> new_layer = tf.keras.layers.Dense(10)
  + >>> print(new_layer.name)
  + dense
  + >>> print(tf.keras.backend.learning_phase())
  + 0
  """
  global _SESSION
  global _GRAPH_LEARNING_PHASES # pylint: disable=global-variable-not-assigned
  