Method add_node

engine/src/execution_graph/graph.cpp:197–204

	size_t graph::num_nodes() const { return kernels_.size(); }

	size_t graph::add_node(std::shared_ptr<kernel> k) {
		if(k != nullptr) {
			container_[k->get_id()] = k;
			kernels_.push_back(k.get());
			return k->get_id();
		}
		return head_id_;
	}

	void graph::add_edge(std::shared_ptr<kernel> source,
		std::shared_ptr<kernel> target,