add test for autodiff about forwarding func

test/forward/2single_variable_function_with_custom_scalar.cpp

@@ -0,0 +1,32 @@
+// NOTE: 使用自定义标量（复数）的单变量函数的导数
+
+// C++ includes
+#include <iostream>
+#include <complex>
+using namespace std;
+
+// autodiff include
+#include <autodiff/forward/dual.hpp>
+using namespace autodiff;
+
+// Specialize isArithmetic for complex to make it compatible with dual
+namespace autodiff::detail {
+    template<typename T>
+    struct ArithmeticTraits<complex<T>>: ArithmeticTraits<T> {};
+} // autodiff::detail
+
+using cxdual = Dual<complex<double>, complex<double>>;
+
+cxdual f(cxdual x){
+    return 1+x+x*x+1/x+log(x);
+}
+
+int main(){
+    cxdual x = 2.0; // the input variable x
+    cxdual u = f(x); // the output variable u
+    cxdual dudx = derivative(f, wrt(x), at(x)); // evaluate the derivative du/dx
+
+    cout << "u = " << u << endl;
+    cout << "du/dx = " << dudx << endl;
+    return 0;
+}