NumNodes in FlowGraphNode is returning wrong value

[shivramsrivastava]

NumNodes in flowgraph is returning wrong value.
we should get the size of node_map_ as the numNodes , in FlowGraph NumNodes is returning current_id_ that is returning wrong value.

there is comment about work around, but work around also giving wrong value.

Note :- when adding Node, current_id_ is increased but not decreasing when deleting the Node/
Note:- node_map_.size() should be used to get exact nodes in the graph.