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/tests/test_saver.py

@@ -0,0 +1,42 @@
+import pytest
+
+from rl_coach.saver import Saver, SaverCollection
+
+
+@pytest.mark.unit_test
+def test_checkpoint_collection():
+    class SaverTest(Saver):
+        def __init__(self, path):
+            self._path = path
+            self._count = 1
+
+        @property
+        def path(self):
+            return self._path
+
+        def merge(self, other: 'Saver'):
+            assert isinstance(other, SaverTest)
+            assert self.path == other.path
+            self._count += other._count
+
+    # test add
+    savers = SaverCollection(SaverTest('123'))
+    savers.add(SaverTest('123'))
+    savers.add(SaverTest('456'))
+
+    def check_collection(mul):
+        paths = ['123', '456']
+        for c in savers:
+            paths.remove(c.path)
+            if c.path == '123':
+                assert c._count == 2 * mul
+            elif c.path == '456':
+                assert c._count == 1 * mul
+            else:
+                assert False, "invalid path"
+
+    check_collection(1)
+
+    # test update
+    savers.update(savers)
+    check_collection(2)