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/saver.py

@@ -0,0 +1,112 @@
+"""
+Module for abstract base class for checkpoint object and checkpoint collection
+"""
+from typing import Any, Dict, List
+
+
+class Saver(object):
+    """
+    ABC for saver objects that implement saving/restoring to/from path, and merging two savers.
+    """
+    @property
+    def path(self):
+        """
+        Relative path for save/load. If two saver objects return the same path, they must be merge-able.
+        """
+        raise NotImplementedError
+
+    def save(self, sess: Any, save_path: str) -> List[str]:
+        """
+        Save to save_path
+        :param sess: active session for session-based frameworks (e.g. TF)
+        :param save_path: full path to save checkpoint (typically directory plus self.path plus checkpoint count).
+        :return: list of all saved paths
+        """
+        raise NotImplementedError
+
+    def restore(self, sess: Any, restore_path: str) -> None:
+        """
+        Restore from restore_path
+        :param sess: active session for session-based frameworks (e.g. TF)
+        :param restore_path: full path to load checkpoint from.
+        """
+        raise NotImplementedError
+
+    def merge(self, other: 'Saver') -> None:
+        """
+        Merge other saver into this saver
+        :param other: saver to be merged into self
+        """
+        raise NotImplementedError
+
+
+class SaverCollection(object):
+    """
+    Object for storing a collection of saver objects. It takes care of ensuring uniqueness of saver paths
+    and merging savers if they have the same path. For example, if a saver handles saving a generic key/value
+    file for all networks in a single file, it can use a more generic path and all savers of all networks would be
+    merged into a single saver that saves/restores parameters for all networks.
+    NOTE: If two savers have the same path, the respective saver class must support merging them
+    into a single saver that saves/restores all merged parameters.
+    """
+    def __init__(self, saver: Saver = None):
+        """
+        :param saver: optional initial saver for the collection
+        """
+        self._saver_dict = dict()  # type: Dict[str, Saver]
+        if saver is not None:
+            self._saver_dict[saver.path] = saver
+
+    def add(self, saver: Saver):
+        """
+        Add a new saver to the collection. If saver.path is already in the collection, merge
+        the new saver with the existing saver.
+        :param saver: new saver to be added to collection
+        """
+        if saver.path in self._saver_dict:
+            self._saver_dict[saver.path].merge(saver)
+        else:
+            self._saver_dict[saver.path] = saver
+
+    def update(self, other: 'SaverCollection'):
+        """
+        Merge savers from other collection into self
+        :param other: saver collection to update self with.
+        """
+        for c in other:
+            self.add(c)
+
+    def save(self, sess: Any, save_path: str) -> List[str]:
+        """
+        Call save on all savers in the collection
+        :param sess: active session for session-based frameworks (e.g. TF)
+        :param save_path: path for saving checkpoints using savers. All saved file paths must
+        start with this path in their full path. For example if save_path is '/home/checkpoints/checkpoint-01',
+        then saved file paths can be '/home/checkpoints/checkpoint-01.main-network' but not
+        '/home/checkpoints/main-network'
+        :return: list of all saved paths
+        """
+        paths = list()
+        for saver in self:
+            paths.extend(saver.save(sess, "{}.{}".format(save_path, saver.path)))
+        return paths
+
+    def restore(self, sess: Any, restore_path: str) -> None:
+        """
+        Call restore on all savers in the collection
+        :param sess: active session for session-based frameworks (e.g. TF)
+        :param restore_path: path for restoring checkpoint using savers.
+        """
+        for saver in self:
+            restore_path = "{}.{}".format(restore_path, saver.path)
+            saver.restore(sess, restore_path)
+
+    def __iter__(self):
+        """
+        Return an iterator for savers in the collection
+        :return: saver iterator
+        """
+        return (v for v in self._saver_dict.values())
+
+
+