message: Fix doc string: remove the internal references and simplify metric sub-classing example.
diff: @@ -109,28 +109,25 @@ class Metric(Layer):
  Example subclass implementation:
  ```
  - class BinaryTruePositives(Metric):
  - def __init__(self, name='binary_true_positives', dtype=None):
  - super(BinaryTruePositives, self).__init__(name=name, dtype=dtype)
  - self.true_positives = self.add_weight(
  - 'true_positives', initializer=init_ops.zeros_initializer)
  + class BinaryTruePositives(tf.keras.metrics.Metric):
  +
  + def __init__(self, name='binary_true_positives'):
  + super(BinaryTruePositives, self).__init__(name=name)
  + self.true_positives = self.add_weight(name='tp', initializer='zeros')
  def update_state(self, y_true, y_pred, sample_weight=None):
  - y_true = math_ops.cast(y_true, dtypes.bool)
  - y_pred = math_ops.cast(y_pred, dtypes.bool)
  - y_pred, y_true, sample_weight = squeeze_or_expand_dimensions(
  - y_pred, y_true, sample_weight)
  + y_true = tf.cast(y_true, tf.bool)
  + y_pred = tf.cast(y_pred, tf.bool)
  - values = math_ops.logical_and(
  - math_ops.equal(y_true, True), math_ops.equal(y_pred, True))
  - values = math_ops.cast(values, self._dtype)
  + values = tf.logical_and(tf.equal(y_true, True), tf.equal(y_pred, True))
  + values = tf.cast(values, self.dtype)
  if sample_weight is not None:
  - sample_weight = math_ops.cast(sample_weight, self._dtype)
  - values = math_ops.multiply(values, sample_weight)
  - self.true_positives.assign_add(math_ops.reduce_sum(values))
  + sample_weight = tf.cast(sample_weight, self.dtype)
  + values = tf.multiply(values, sample_weight)
  + return self.true_positives.assign_add(tf.reduce_sum(values))
  def result(self):
  - return array_ops.identity(self.true_positives)
  + return tf.identity(self.true_positives)
  ```
  """
  