Policy gradient example: record stats for tensorboard (#577)

* add tf metrics

* comments

* fix network scopes

* add doc

* use format string

* fix trace level

* plot intermediate and final sgd stats

* add back a global step

doc/source/example-policy-gradient.rst

@@ -30,5 +30,15 @@ try passing in the ``Pong-v0`` environment or the ``CartPole-v0`` environment.
 If you wish to use a different environment, you will need to change a few lines
 in ``example.py``.
 
+Current and historical training progress can be monitored by pointing
+TensorBoard to the log output directory as follows.
+
+.. code-block:: bash
+
+  tensorboard --logdir=/tmp/ray
+
+Many of the TensorBoard metrics are also printed to the console, but you might
+find it easier to visualize and compare between runs using the TensorBoard UI.
+
 .. _`TensorFlow with GPU support`: https://www.tensorflow.org/install/
 .. _`code for this example`: https://github.com/ray-project/ray/tree/master/examples/policy_gradient