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/architectures/architecture.py

@@ -19,6 +19,7 @@ from typing import Any, Dict, List, Tuple
 import numpy as np
 
 from rl_coach.base_parameters import AgentParameters
+from rl_coach.saver import SaverCollection
 from rl_coach.spaces import SpacesDefinition
 
 
@@ -213,3 +214,12 @@ class Architecture(object):
         :param placeholder: a placeholder for binding the value to assign_op.
         """
         raise NotImplementedError
+
+    def collect_savers(self, parent_path_suffix: str) -> SaverCollection:
+        """
+        Collection of all savers for the network (typically only one saver for network and one for ONNX export)
+        :param parent_path_suffix: path suffix of the parent of the network
+            (e.g. could be name of level manager plus name of agent)
+        :return: saver collection for the network
+        """
+        raise NotImplementedError