Things will be a bit different when the General Assembly returns to the Statehouse for its fall veto session in October. The weather will be cooler. The days will be shorter. And for the first time in more than 20 years, Kirk W. Dillard will not be a state senator. The Hinsdale Republican and two-time candidate for governor officially stepped down this month to become chairman of the Regional Transportation Authority. A partner at Locke, Lord LLP in Chicago, Dillard said this week that he’s thrown himself …