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/mxnet_components/heads/v_head.py

@@ -98,4 +98,4 @@ class VHead(Head):
         :param x: middleware state representation, of shape (batch_size, in_channels).
         :return: final output of value network, of shape (batch_size).
         """
-        return self.dense(x).squeeze()
+        return self.dense(x).squeeze(axis=1)