Update the constructor constructing an identity matrix

[Kaiser-Yang]

In many libs that constructors constructing a object with specific size will use ELEMENT_TYPE() as its default value, for example, std::vector<int> a(3) will construct a std::vector object whose length is 3 and elements are all 0 (int()). However, in our codes, Matrix<int> a(Shape{3, 3}) will construct an identity matrix which is not consistent with the familiar ways, which will confused users easily.