Many observed President Obama pay homage at the Pentagon yesterday to the nearly 3,000 victims who lost their lives on Sept. 11, 2001. But by nightfall, another commemoration ceremony was gathering — 50 service members joined together to give each rep, each lunge, each workout move in honor of the victims who died 13 years ago.  Army Staff Sgt. River Mitchell,  a chef in the secretary of defense mess at the Pentagon, planned the workout of the day — or  “9/11 Tribute WOD” —  so that his fellow comrades could test their strength on behalf of the fallen in a unique way,…