By Tom Westbrook SINGAPORE (Reuters) -The dollar headed for its biggest monthly gain since September and the yen for its sharpest drop in over a year on Wednesday, as traders waited on a U.S. rates decision to round out January. A sharp slowdown in Australian inflation pushed the Aussie dollar down 0.5% to $0.6567 and rallied bonds as investors pulled forward wagers on interest rate cuts. Elsewhere moves were more modest, and the yen made little immediate reaction to a hawkish tilt at the Bank of Japan, while markets waited to hear from the Federal Reserve. The dollar was slightly firmer at $1…