private void removeDummySourceNode(final Graph<V> residualGraph) {
        if (!residualGraph.isResidualGraph) {
            throw new IllegalStateException("Graph should be residual graph to remove dummy source node");
        }
        residualGraph.adjList.remove(null);
        residualGraph.nodes.remove(null);
    }
Get residual graph of this graph.
Residual graph definition:
If there is an edge in original graph from u to v with capacity c, cost w and flow f,
then in the new graph there are two edges e1 and e2. e1 is from u to v with capacity c - f,
cost w and flow f. e2 is from v to u with capacity f, cost -w and flow 0.

@return Residual graph
streams/src/main/java/org/apache/kafka/streams/processor/internals/assignment/Graph.java