Fixed unnecessary allocs for softmax
Fixed unnecessary allocs for softmax

mitchellstern/dynet,Peilun-Li/dynet,kamigaito/cnn,Peilun-Li/dynet,cherryc/dynet,Abasyoni/dynet,cherryc/dynet,Abasyoni/dynet,Abasyoni/dynet,kamigaito/cnn,cherryc/dynet,mitchellstern/dynet,Peilun-Li/dynet