I doubt Naruto and Sakura are already couples in the movie, that would be too much trolling.
Who's to say that they won't be dating, though, at the very least? I suspect that they are... I don't know. There's just something about the way their casual clothes seem to complement one another that makes it seem that they're a couple. While I'm not artist, myself, I definitely can catch how some things are designed to just go together, which might have an implication for the bigger picture. Maybe I'm overthinking it?