By Herbert Lash and Danilo Masoni NEW YORK/MILAN (Reuters) -World stocks slid further and Treasury yields dipped on Wednesday as U.S. worker productivity data beat forecasts but extended a weak trend, further muddying a debate on how far and how fast U.S. interest rates will rise. Third quarter productivity rebounded at a slightly faster pace than initially thought. Economists said the reading pointed to elevated labor costs and inflation staying high, adding pressure on the Federal Reserve to keep raising rates. But benchmark U.S. yields and the dollar both fell, suggesting lower rates ahead….