The WBZ-TV mayoral debate between Mayor Thomas Menino and his three challengers has been pushed back a week to Sept. 2, two campaigns said Wednesday. Same time: 7 p.m. And same place: at the station's Allston studios on Soldiers Field Road. There's also a 6:30 p.m. forum, sponsored by MassVOTE and a host of other organizations, scheduled for Sept. 3 at Reggie Lewis Center in Roxbury. (Menino has been non-committal on the MassVOTE forums, saying he has already committed to three debates and a WCVB forum.)