nnvm graph

class Graph {
 public:
  // outputs of the computation graph.
  std::vector<NodeEntry> outputs;
  std::unordered_map<std::string, std::shared_ptr<any> > attrs;

 private:
  // internal structure of indexed graph
  std::shared_ptr<const IndexedGraph> indexed_graph_;
};

class IndexedGraph {
 private:
  // Node pointers in CSR structure.
  std::vector<Node> nodes_;
  // Index to all input nodes.
  std::vector<uint32_t> input_nodes_;
  // Index to all mutable input nodes.
  std::unordered_set<uint32_t> mutable_input_nodes_;
  // space to store the outputs entries
  std::vector<NodeEntry> outputs_;
  // mapping from node to index.
  std::unordered_map<const nnvm::Node*, uint32_t> node2index_;
  // CSR pointer of node entries
  std::vector<size_t> entry_rptr_;
  // space to store input entries of each
  std::vector<NodeEntry> input_entries_;
  // control flow dependencies
  std::vector<uint32_t> control_deps_;
};