THE Hamptons season goes out with a bang Labor Day Weekend. Sunday Sept. l being Grand Prix Day at the Hampton Classic. Such rules and regs hoo-hah. Credentials at the Will Call Window. Enter Show Grounds at Garden Entrance. Proceed to Glass Conservatory on the left, etc. That's lunch. Dinner's at Denise Rich's rich home. This comes with cocktails, Liza Minnelli In Person singing, a $25,000 Diane Johnson necklace and $17,000 Aaron Basha diamond ring to be auctioned. Proceeds to the G&P Foundation for Cancer Research in memory of Denise's daughter. Gabrielle. For tickets, call (917) 351-8600.