message: Create docstring for poisson()
diff: @@ -901,6 +901,28 @@ def kullback_leibler_divergence(y_true, y_pred): # pylint: disable=missing-docs
  @keras_export('keras.metrics.poisson', 'keras.losses.poisson')
  def poisson(y_true, y_pred):
  + """Computes the Poisson loss between y_true and y_pred.
  +
  + The Poisson loss is the mean of the elements of the `Tensor`
  + `y_pred - y_true * log(y_pred)`.
  +
  + Usage:
  +
  + ```python
  + loss = tf.keras.losses.poisson([1.4, 9.3, 2.2], [4.3, 8.2, 12.2])
  + print('Loss: ', loss.numpy()) # Loss: -0.8045559
  + ```
  +
  + Args:
  + y_true: tensor of true targets.
  + y_pred: tensor of predicted targets.
  +
  + Returns:
  + A `Tensor` with the mean Poisson loss.
  +
  + Raises:
  + InvalidArgumentError: If `y_true` and `y_pred` have incompatible shapes.
  + """
  y_pred = ops.convert_to_tensor(y_pred)
  y_true = math_ops.cast(y_true, y_pred.dtype)
  return K.mean(y_pred - y_true * math_ops.log(y_pred + K.epsilon()), axis=-1)
  