Base optimizer tracking (tensorflow#2126)

* Update lookahead.py

Inital fix of 
tensorflow#2094
tensorflow#2102

* Fix linting

* Resolve name conflict with mixed prexision

* Track baseline optimizer in avg

Author: bhack

tensorflow_addons/optimizers/average_wrapper.py

@@ -46,6 +46,7 @@ def __init__(
             raise TypeError("sequential_update must be of bool type")
 
         self._optimizer = optimizer
+        self._track_trackable(self._optimizer, "awg_optimizer")
 
         if sequential_update is not None:
             warnings.warn(

tensorflow_addons/optimizers/lookahead.py

@@ -80,6 +80,7 @@ def __init__(
         self._set_hyper("sync_period", sync_period)
         self._set_hyper("slow_step_size", slow_step_size)
         self._initialized = False
+        self._track_trackable(self._optimizer, "lh_base_optimizer")
 
     def _create_slots(self, var_list):
         self._optimizer._create_slots(