Knight Trading Group Warns of Loss  NEW YORK (Reuters) - Knight Trading Group Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=NITE.O target=/stocks/quickinfo/fullquote"&gt;NITE.O&lt;/A&gt;,  one of the biggest traders of Nasdaq stocks, on Wednesday  forecast a third-quarter loss, missing Wall Street expectations  of a profit, because of low trading volumes.