Method init

dnn/src/naive/pooling/opr_impl.cpp:23–26

    bool fed;
    MaxPooler(size_t, DType) : answer(DTypeTrait<ctype>::min()) {}
    void init() {
        answer = DTypeTrait<ctype>::min();
        fed = false;
    }
    void feed(ctype x) {
        answer = answer > x ? answer : x;
        fed = true;