message: feature-gate AD optimizations behind flag
diff: @@ -196,7 +196,8 @@ struct Variable {
  T z = 0.f, v2 = select(eq(v1, z), z, v2_);
  if constexpr (is_jit_array_v<T>) {
  - if (v2_.is_literal() && std::isnormal(v2_[0]))
  + if (v2_.is_literal() && std::isnormal(v2_[0]) &&
  + jit_flag(JitFlag::ADOptimize))
  v2 = v2_;
  }
  