forward-mode jacobian aborts for mappings R^n -> R^m with m != n

[sims1253]

The fwd jacobian sizes both J and fx from the input dimension before evaluating f; J.col(0) = fx_fvar.d() then assigns an m-vector into an n-row column:

struct F {  // R^2 -> R^3
  template <typename T>
  Eigen::Matrix<T, Eigen::Dynamic, 1> operator()(
      const Eigen::Matrix<T, Eigen::Dynamic, 1>& x) const {
    Eigen::Matrix<T, Eigen::Dynamic, 1> out(3);
    out << x(0), x(0) + x(1), x(0) * x(1);
    return out;
  }
};
stan::math::jacobian(F{}, x, fx, J);   // x = [1.5, 2.0]
// fwd: Eigen assertion "DenseBase::resize() does not actually allow to resize." -> abort

The rev jacobian sizes from the output and handles the identical functor correctly (fx = 3-vector, J = 3×2). With assertions off, the fwd version writes past the storage instead of aborting.

Probably needs to evaluate f first and size from fx_fvar.size() simliar to the rev implementation.