Rosenbrock function example for auto-differentiation

Rosenbrock.cpp

// Rosenbrock function, a = 1.0, b = 100
class Rosenbrock {
public:
  template <typename T>
  bool operator()(const T *const x, const T *const y, T *cost) const {
    cost[0] = (T(1.0) - x[0]) * (T(1.0) - x[0]) +
              T(100.0) * (y[0] - x[0] * x[0]) * (y[0] - x[0] * x[0]);
    return true;
  }
};