She won't run in 2020... But she will be a Senator by the end of 2018. Its a win-win-win for Graham-Haley-Trump... with no drawbacks (scenario below).
I would bet a lot...
that she and Graham (and Trump) have worked out an agreement. Graham will take a cabinet position- lately it seems likely to be Secretary of Defense. Haley will be appointed Senator, but will not run for the Full Term in 2020. She will just fulfill her place holder- and will get out so she can announce her Presidential Bid in 2022 (without looking too opportunistic of running for President 2 years after winning a Senate election).
One reason to resign from UN early... is it will be more palatable for her to be appointed Senator if she is living back in SC (even for a month or so), and not moving directly from one appointment to another. By resigning is looks more like she is done with her UN service and wanted to move back to her home state, etc.
Graham gets a 2 year hiatus in a major Cabinet position, then get to go back to his Senate seat. Haley gets to add Senator to her Governor/ UN Ambassador resume, and Trump gets a Cabinet Secretary he has some control over & gets a strong potential candidate in 2024 (in case he wins re-election, he will want his successor to be a Republican- which will look like a victory for his presidency. And he knows/ thinks ... that Haley will be much strong candidate than Mike Pence).
I wouldn't be shocked if Trump didn't run in 2020, but I doubt Haley would be the nominee. She may well be a VP nominee, however; she's the Trump appointee whose stock has gone up the most since being appointed.
The Above Scenario was the past for her in 2024... not 2020. It was saying why she will be appointed Senator in the next month or two... and how such a scenario would be a win-win-win for Haley- Lindsay Graham- Trump.