Adding checkpointing framework (#74)

* Adding checkpointing framework as well as mxnet checkpointing implementation.

- MXNet checkpoint for each network is saved in a separate file.

* Adding checkpoint restore for mxnet to graph-manager

* Add unit-test for get_checkpoint_state()

* Added match.group() to fix unit-test failing on CI

* Added ONNX export support for MXNet

rl_coach/level_manager.py

@@ -20,6 +20,7 @@ from rl_coach.agents.composite_agent import CompositeAgent
 from rl_coach.core_types import EnvResponse, ActionInfo, RunPhase, ActionType, EnvironmentSteps, Transition
 from rl_coach.environments.environment import Environment
 from rl_coach.environments.environment_interface import EnvironmentInterface
+from rl_coach.saver import SaverCollection
 from rl_coach.spaces import ActionSpace, SpacesDefinition
 
 
@@ -292,3 +293,13 @@ class LevelManager(EnvironmentInterface):
 
     def should_stop(self) -> bool:
         return all([agent.get_success_rate() >= self.environment.get_target_success_rate() for agent in self.agents.values()])
+
+    def collect_savers(self) -> SaverCollection:
+        """
+        Calls collect_savers() on all agents and combines the results to a single collection
+        :return: saver collection of all agent savers
+        """
+        savers = SaverCollection()
+        for agent in self.agents.values():
+            savers.update(agent.collect_savers(parent_path_suffix=self.name))
+        return savers