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/tensorflow_components/architecture.py

@@ -23,6 +23,7 @@ import tensorflow as tf
 from rl_coach.architectures.architecture import Architecture
 from rl_coach.base_parameters import AgentParameters, DistributedTaskParameters
 from rl_coach.core_types import GradientClippingMethod
+from rl_coach.saver import SaverCollection
 from rl_coach.spaces import SpacesDefinition
 from rl_coach.utils import force_list, squeeze_list, start_shell_command_and_wait
 
@@ -637,6 +638,16 @@ class TensorFlowArchitecture(Architecture):
             self.curr_rnn_c_in = self.middleware.c_init
             self.curr_rnn_h_in = self.middleware.h_init
 
+    def collect_savers(self, parent_path_suffix: str) -> SaverCollection:
+        """
+        Collection of all checkpoints for the network (typically only one checkpoint)
+        :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: checkpoint collection for the network
+        """
+        # TODO implement returning checkpoints for tensorflow
+        return SaverCollection()
+
 
 def save_onnx_graph(input_nodes, output_nodes, checkpoint_save_dir: str) -> None:
     """