Created by: Xreki

In some operators, we may need attributes of which the type is std::vector<int>. We can define them in C++ code as follows:

    AddAttr<std::vector<int>>("xNumColDims", "Some comments");
    AddAttr<std::vector<int>>( "wNumColDims", "Some comments");

So, the problem is: we don't know the length in advance, "how to set the default value".