Drop decltype and forward

Author: andrjohns

stan/math/prim/functor/finite_diff_gradient_auto.hpp

@@ -62,11 +62,11 @@ void finite_diff_gradient_auto(const F& f, VectorT&& x, ScalarT& fx,
     double h = finite_diff_stepsize(value_of_rec(x[i]));
     ScalarT delta_f = 0;
     for (int j = 0; j < 6; ++j) {
-      decltype(auto) x_temp
+      auto x_temp
           = EigT::NullaryExpr(x.size(), [&x, &i, &h, &j](Eigen::Index k) {
               return k == i ? x[i] + h * h_scale[j] : x[k];
             });
-      delta_f += f(std::forward<decltype(x_temp)>(x_temp)) * mults[j];
+      delta_f += f(std::move(x_temp)) * mults[j];
     }
     return delta_f / (60 * h);
   });