fix expm1 return type

Author: SteveBronder

stan/math/rev/fun/expm1.hpp

@@ -42,8 +42,8 @@ inline var expm1(const var& a) {
 }
 
 template <typename T, require_matrix_t<T>* = nullptr>
-inline var expm1(const var_value<T>& a) {
-  return make_callback_var(expm1(a.val()), [a](auto& vi) mutable {
+inline auto expm1(const var_value<T>& a) {
+  return make_callback_var(expm1(a.val()).eval(), [a](auto& vi) mutable {
     a.adj().array() += vi.adj().array() * (vi.val().array() + 1.0);
   });
 }