I don’t think it’s ever been tested*, but arguably no, the president can’t pardon war crimes. They can only pardon “offenses against the United States”. A war crime isn’t just an offense against the United States but also a violation of international law. So, a future administration could try to go after these people for the international law violations. As others have noted, the chances of that actually happening are nil. *Trump has pardoned war crimes, but no one has yet tried going after the war criminals for the international law violations they committed.