In the strictest sense, all fan fiction is AU because it steps outside the canon universe envisioned by the creators.
Having said that, I take the personal opinion that by staying within the general canon, in terms of keeping canon characters in character, and using the conventions and settings of the original creators of the 'verse, that fan fiction isn't AU. We're playing in the same sandbox, so to speak.
Celebrity fiction, or any real-person fiction, can be held to that looser standard, I think. It's taking the people we purport to admire, and placing them, still as themselves, in a setting that is totally fictitious, but not outside the realm of possibility for that celebrity. However, unless we take a celebrity and put them someplace wildly outside the realm of possibility - Jennifer Lawrence is piloting a mining ship in the asteroid belt - it really isn't AU, no matter how out of character it might seem for Mira Sorvino to be elected President some day, because really, it could happen. I'd hope she were wiser than to want the job, but it's not impossible.