(Reuters) - Traders see the Federal Reserve starting a round of interest rates cuts this month with an upsized 50 basis point reduction, after a government report showed the unemployment rate eased to 4.2% in August but employers added fewer workers than economists had expected in both August and July.    Traders now see a 55% chance that the Fed will cut its policy rate, now in the 5.25%-5.50% range, to a 4.75%-5% range at its upcoming meeting on Sept 17-18. Before the report they had seen about a 43% chance of that outcome, favoring instead a quarter-point reduction