message: Updates the docstring and model examples to use new style to specify metric_fn.
diff: @@ -980,18 +980,20 @@ class TPUEstimator(estimator_lib.Estimator):
  Example (MNIST):
  ```
  - def model_fn(features, labels, mode, config, params):
  - ...
  - logits = ...
  -
  - if mode = tf.estimator.ModeKeys.EVAL:
  + # The metric Fn which runs on CPU.
  def metric_fn(labels, logits):
  predictions = tf.argmax(logits, 1)
  return {
  - 'precision': tf.metrics.precision(
  + 'accuracy': tf.metrics.precision(
  labels=labels, predictions=predictions),
  }
  + # Your model Fn which runs on TPU.
  + def model_fn(features, labels, mode, config, params):
  + ...
  + logits = ...
  +
  + if mode = tf.estimator.ModeKeys.EVAL:
  return tpu_estimator.TPUEstimatorSpec(
  mode=mode,
  loss=loss,
  