Method to_string

dnn/src/common/utils.cpp:320–326

======================== CpuNDRange ======================== */

/* ======================== CpuNDRange ======================== */
std::string CpuNDRange::to_string() const {
    std::string ret;
    for (size_t i = 0; i < m_dimension; i++) {
        ret += megdnn::ssprintf(" %zu", m_dim[i]);
    }
    return ret;
}

size_t& CpuNDRange::operator[](size_t idx) {
    megdnn_assert(