message: throw GKO_NOT_IMPLEMENT when cusparse doesn't support the type
diff: @@ -727,6 +727,8 @@ void spmv(std::shared_ptr<const CudaExecutor> exec,
  cusparseSetPointerMode(handle, CUSPARSE_POINTER_MODE_DEVICE));
  cusparse::destroy(descr);
  + } else {
  + GKO_NOT_IMPLEMENTED;
  }
  }
  }
  