Method build

imperative/python/src/ops.cpp:582–588

        std::shared_ptr<mgb::Hashable> key = nullptr;

        std::shared_ptr<OpDef> build() {
            if (key == nullptr) {
                key = std::make_shared<UniqueKey>();
            }
            return SubgraphOp::make(
                    name, std::make_shared<Subgraph>(graph), output_grad_mask, key);
        }
    };

    py::class_<PySubgraphBuilder>(m, "SubgraphBuilder")