WASHINGTON, May 21 (Reuters) - U.S. Defense Secretary Pete Hegseth met billionaire Elon Musk at the Pentagon on Wednesday, officials said, the second known time the close ally of President Donald Trump has visited the department's headquarters.In March, Musk, whose businesses have a number of Defense Department contracts, was granted an unprecedented top-level meeting at the Pentagon."The Secretary met with Elon Musk and other members of the X AI team this morning," Pentagon spokesperson Sean Parnell said in a statement."The Department of Defense is committed to engaging with AI industry CEOs to ensure our warfighters are equipped to face 21st...