Add graceful restore

[eywalker]

Currently when you try to restore the content of the checkpoint, it will try to restore every variable defined in the current graph. If the checkpoint file is missing some of the variable present in the current graph, then the restore will fail. Although this is the expected behavior and works fine, sometimes you want to restore some variables from a different graph. To make this process easier, we should add "graceful" restore where we only restore variables in the graph if it is present in the checkpoint.