Brian Peterson — the privileged New Jersey youth who made headlines in 1996 when he and his girlfriend Amy Grossberg killed their newborn baby — is out of jail, married to another woman, and living in Florida. He spoke to The Post recently as he and his wife Jaime took a stroll in the town of Jupiter with their two dogs — and showed he was in no way marking the short life of his son. "It was seven years ago," Brian Peterson said when approached outside his swank townhouse. The significant date to him was apparently July 1998 —...