Nelly -- Huge Stars Hit 40th Bday Party ... Including Championship Boxer

Nelly turned 40 this weekend ... and some HUGE stars came out to his L.A. mansion to celebrate -- including big-time actors, producers and a major boxing champ.

Former "Scandal" star Columbus Short was there ... along with Nick Cannon, "Real Husbands of Hollywood" star Duane Martin and boxing champ Andre Berto.

We're told the whole thing was put together by Nelly's GF Shantel Jackson (who was formerly engaged to Floyd Mayweather) ... and included a private chef, champagne and even a pinata (see the video below).

Sources tell us ... Berto is still friendly with Mayweather (who has a beef with Nelly) ... but we're told Nelly doesn't care and has never put him in a "pick sides" situation.

Pretty mature for a 40-year-old.