  PyObject* AggregateGradients(
      tensorflow::gtl::ArraySlice<PyObject*> gradient_tensors) const final {
    PyObject* list = PyList_New(gradient_tensors.size());
    for (int i = 0; i < gradient_tensors.size(); ++i) {
      // Note: stealing a reference to the gradient tensors.
      CHECK(gradient_tensors[i] != nullptr);
      CHECK(gradient_tensors[i] != Py_None);
      PyList_SET_ITEM(list, i,
                      reinterpret_cast<PyObject*>(gradient_tensors[i]));
    }
    PyObject* arglist = Py_BuildValue("(O)", list);
    CHECK(arglist != nullptr);
    PyObject* result = PyEval_CallObject(aggregate_fn_, arglist);
    Py_DECREF(arglist);
    Py_DECREF(list);
    return result;
  }
Unknown