Make attribute support for std::vector<std::pair<int, int>>

[wanghaoshuang]

Pad op need a attribute named paddings which is std::vector<std::pair<int, int>>.
So OperatorBase::AddAttr(...) should support for std::vector<std::pair<int, int>>.

[reyoung]

@wanghaoshuang @Canpio @qingqing01

I do not think this type of attribute is necessary since we could use four attributes in this solution, pad_left, pad_right, pad_top, pad_bottom. Moreover, if we add type, a list of int pairs, what about float pairs? string pairs? int tuples? (list<pair<float, float>>, list<pair<str, str>>, list<tuple<int, int, int>>)

If we adding list<pair<int, int>>, it is a trending to make that attribute more and more complex. I don't think it is a good choice.

[wanghaoshuang]

@reyoung
pad_left, pad_right, pad_top, pad_bottom just suits 2-D tensor. Yes, we can use vector<int> instead of vector<pair<int, int>>. Which need developer writting python code to transfer list<tuple> to vector<int> and cpp code to transfer vector<int> to vector<pair<int, int>>.