Improve sub-graph implementation #201
Description
When adding an executable graph as a subgraph of a parent graph, we don't take a copy of the sub-graph object. This results in the root & exit nodes of the subgraph having predecessors and successors set on them. This update to the graph structure affects standalone
execution of the executable graph added as a subgraph, such that its execution is incorrect. Adding the subgraph more than once to parent graphs also doesn't result in correct execution, as the root/exit nodes of the sub-graph can no longer be determined correctly.
See the tests added in #202 which are marked as XFAIL for use-cases that fail and should be fixed.