WASHINGTON — The Federal Reserve on Wednesday left its benchmark interest rate unchanged while signaling further gradual rate hikes in the months ahead as long as the economy stays healthy. The Fed’s widely expected decision kept the central bank’s key short-term rate at 1.75 percent to 2 percent – the level hit in June when the Fed boosted the rate for a second time this year. In June, the Fed projected four rate hikes this year, up from three in 2017.