add test for autodiff about forwarding func

test/forward/3multi_varialbe_function.cpp

@@ -0,0 +1,32 @@
+// NOTE: 多变量函数的导数
+
+// C++ includes
+#include <iostream>
+
+// autodiff include
+#include <autodiff/forward/dual.hpp>
+using namespace autodiff;
+
+// The multi-variable function for which derivatives are needed
+dual f(dual x, dual y, dual z)
+{
+    return 1 + x + y + z + x*y + y*z + x*z + x*y*z + exp(x/y + y/z);
+}
+
+int main()
+{
+    dual x = 1.0;
+    dual y = 2.0;
+    dual z = 3.0;
+
+    dual u = f(x, y, z);
+
+    double dudx = derivative(f, wrt(x), at(x, y, z));
+    double dudy = derivative(f, wrt(y), at(x, y, z));
+    double dudz = derivative(f, wrt(z), at(x, y, z));
+
+    std::cout << "u = " << u << std::endl;         // print the evaluated output u = f(x, y, z)
+    std::cout << "du/dx = " << dudx << std::endl;  // print the evaluated derivative du/dx
+    std::cout << "du/dy = " << dudy << std::endl;  // print the evaluated derivative du/dy
+    std::cout << "du/dz = " << dudz << std::endl;  // print the evaluated derivative du/dz
+}