add test for autodiff about forwarding func

test/forward/6gradient_vector_of_scalar_func.cpp

@@ -0,0 +1,29 @@
+// NOTE: 标量函数的梯度向量
+// C++ includes
+#include <iostream>
+
+// autodiff include
+#include <autodiff/forward/real.hpp>
+#include <autodiff/forward/real/eigen.hpp>
+using namespace autodiff;
+
+// The scalar function for which the gradient is needed
+real f(const ArrayXreal& x)
+{
+    return (x * x.exp()).sum(); // sum([xi * exp(xi) for i = 1:5])
+}
+
+int main()
+{
+    using Eigen::VectorXd;
+
+    ArrayXreal x(5);                            // the input array x with 5 variables
+    x << 1, 2, 3, 4, 5;                         // x = [1, 2, 3, 4, 5]
+
+    real u;                                     // the output scalar u = f(x) evaluated together with gradient below
+
+    VectorXd g = gradient(f, wrt(x), at(x), u); // evaluate the function value u and its gradient vector g = du/dx
+
+    std::cout << "u = " << u << std::endl;      // print the evaluated output u
+    std::cout << "g = \n" << g << std::endl;    // print the evaluated gradient vector g = du/dx
+}