message: Add example for LR scheduler
  A short and simple example, especially for new users. It helps for a general understanding on how to use this callback. Open for feedback and suggestions. ;)
diff: @@ -1168,6 +1168,23 @@ class LearningRateScheduler(Callback):
  (integer, indexed from 0) and returns a new
  learning rate as output (float).
  verbose: int. 0: quiet, 1: update messages.
  +
  + ```python
  + # Firstly, let's create a function which
  + # maps a given epoch to a learning rate.
  + # This function would keep the learning rate
  + # constant at 0.001 for the first ten epochs and
  + # let it decrease exponentially after that.
  + def scheduler(epoch):
  + if epoch < 10:
  + return 0.001
  + else:
  + return 0.001 * tf.math.exp(0.1 * (10 - epoch))
  + # Next, we need to set up the callback and train the model.
  + callback = tf.keras.callbacks.LearningRateScheduler(scheduler)
  + model.fit(data, labels, epochs=100, callbacks=[callback],
  + validation_data=(val_data, val_labels))
  + ```
  """
  def __init__(self, schedule, verbose=0):
  