By Purvi Agarwal and Sukriti Gupta (Reuters) - Wall Street futures were flat to slightly lower on Wednesday, as markets were guarded ahead of a widely anticipated interest-rate cut from the Federal Reserve later in the day, and Nvidia dipped after a report Chinese tech firms might stop buying its chips. The U.S. central bank is expected to reduce borrowing costs by at least 25 basis points at 2 p.m. ET, a move priced in by investors after a series of economic indicators indicated a weakening jobs market. ...