add test for autodiff about forwarding func

test/forward/8jacobian_matrix_of_vector_func.cpp

@@ -0,0 +1,32 @@
+// NOTE: 向量函数的雅可比矩阵 
+
+// C++ includes
+#include <iostream>
+
+// autodiff include
+#include <autodiff/forward/real.hpp>
+#include <autodiff/forward/real/eigen.hpp>
+using namespace autodiff;
+
+// The vector function for which the Jacobian is needed
+VectorXreal f(const VectorXreal& x)
+{
+    return x * x.sum();
+}
+
+int main()
+{
+    using Eigen::MatrixXd;
+
+    VectorXreal x(5);                           // the input vector x with 5 variables
+    x << 1, 2, 3, 4, 5;                         // x = [1, 2, 3, 4, 5]
+
+    VectorXreal F;                              // the output vector F = f(x) evaluated together with Jacobian matrix below
+    MatrixXd J1 = jacobian(f, wrt(x), at(x), F); // evaluate the output vector F and the Jacobian matrix dF/dx
+
+    std::cout << "F = \n" << F << std::endl;    // print the evaluated output vector F
+    std::cout << "J1 = \n" << J1 << std::endl;    // print the evaluated Jacobian matrix dF/dx
+
+    MatrixXd J2 = jacobian(f, wrt(x), at(x));
+    std::cout << "J2 = \n" << J2 << std::endl;    // print the evaluated Jacobian matrix dF/dx
+}