[tune] PBT + Memnn example (#5723)

* Add example file

* Move into train function

* Somewhat working example of MemNN, still has some failed trials

* Reorganize into a class

* Small fixes

* Iteration decrease and fix hyperparam_mutations

* Add example file

* Move into train function

* Somewhat working example of MemNN, still has some failed trials

* Reorganize into a class

* Small fixes

* Iteration decrease and fix hyperparam_mutations

* Some style edits

* Address PR changes without modifying learning rate

* Add configs and hyperparameter mutations

* Add tune test

* Modify import locations

* Some parameter changes for testing

* Update memnn example

* Add tensorboard support and address PR comment

* Final changes

* lint

* generator

doc/source/tune-examples.rst

@@ -16,6 +16,7 @@ General Examples
 - `pbt_example <https://github.com/ray-project/ray/blob/master/python/ray/tune/examples/pbt_example.py>`__: Example of using a Trainable class with PopulationBasedTraining scheduler.
 - `pbt_ppo_example <https://github.com/ray-project/ray/blob/master/python/ray/tune/examples/pbt_ppo_example.py>`__: Example of optimizing a distributed RLlib algorithm (PPO) with the PopulationBasedTraining scheduler.
 - `logging_example <https://github.com/ray-project/ray/blob/master/python/ray/tune/examples/logging_example.py>`__: Example of custom loggers and custom trial directory naming.
+- `pbt_memnn_example <https://github.com/ray-project/ray/blob/master/python/ray/tune/examples/pbt_memnn_example.py>`__: Example of training a Memory NN on bAbI with Keras using PBT.
 
 Search Algorithm Examples
 -------------------------