message: Update docs for `tf.contrib.losses` -> `tf.losses`
  This fix updates the docs in `extend/estimators.md`
  and changes the reference from `tf.contrib.losses`
  to `tf.losses`, as the former has been deprecated.
diff: @@ -515,7 +515,7 @@ using `mean_squared_error()` (in bold):
  loss = tf.losses.mean_squared_error(labels, predictions)</strong>
  ...</code></pre>
  -See the @{$python/contrib.losses$API guide} for a
  +See the @{tf.losses$API guide} for a
  full list of loss functions and more details on supported arguments and usage.
  Supplementary metrics for evaluation can be added to an `eval_metric_ops` dict.
  @@ -694,5 +694,5 @@ For additional reference materials on building `Estimator`s, see the following
  sections of the API guides:
  * @{$python/contrib.layers$Layers}
  -* @{$python/contrib.losses$Losses}
  +* @{tf.losses$Losses}
  * @{$python/contrib.layers#optimization$Optimization}
  