Debonair “Downton Abbey” star Dan Stevens has become the toast of New York during his run in “The Heiress” with Jessica Chastain. We hear the thespian will be feted at a dinner next month at John DeLucie’s new restaurant Bill’s, thrown by Town & Country’s Jay Fielden. Stevens is profiled in the latest issue, out Tuesday, in which series creator Julian Fellowes says he discovered the actor in a lesser role in a BBC production of “Sense and Sensibility.” “If you can steal ‘Sense and Sensibility’ as Edward Ferrars, one of the dullest parts in the English canon, you have definitely got something,” Fellowes quips. Meanwhile, Vanity Fair editor Graydon Carter and Ralph Lauren are hosting a premiere for the third season of “Downton Abbey” at MoMA on Monday. The only problem with Stevens’ success — for diehard “Downton” fans, at least — is he’s reportedly leaving the show to pursue other projects. Fellowes hinted at the situation in a recent report, and said he won’t recast the role of Matthew Crawley if Stevens doesn’t sign for a new season.