fix fwd tests (remove const)

Author: Jouni Helske

test/unit/math/rev/fun/log_softmax_test.cpp

@@ -73,7 +73,7 @@ TEST(AgradRev, log_softmax_negative_infinity_has_finite_adjoint) {
 
   var_value<Eigen::VectorXd> x(v);
 
-  const auto y = log_softmax(x);
+  auto y = log_softmax(x);
   sum(y).grad();
 
   const double e1 = std::exp(1.0);

test/unit/math/rev/fun/log_sum_exp_test.cpp

@@ -73,7 +73,7 @@ TEST(AgradRev, log_sum_exp_negative_infinity_has_zero_adjoint) {
 
   var_value<Eigen::VectorXd> x(v);
 
-  const auto y = log_sum_exp(x);
+  auto y = log_sum_exp(x);
   y.grad();
 
   const double e1 = std::exp(1.0);