speed up some common DenseTensor2 dot cases
speed up some common DenseTensor2 dot cases

pogren-oracle/factorie,joprice/factorie,digitalreasoning/factorie-scala-210,melisabok/factorie,malcolmgreaves/factorie,zxsted/factorie,strubell/factorie,patverga/factorie,asubbaswamy/factorie,Craigacp/factorie,factorie/factorie,hlin117/factorie