MIAMI — A U.S. Customs and Border Protection agent, teaching a gun instruction class at a West Miami-Dade County gun range, was shot Wednesday morning and was pronounced dead at the hospital where doctors were unable to revive him. Details of the shooting weren’t immediately clear but sources familiar with the incident said it was accidental. CBP Spokeswoman Rachel Torres confirmed one of her agents had been shot at the range and died of his injuries at the hospital. “We did have a CBP officer critically wounded this morning. He was teaching a class,” she said. Several law enforcement sources …