simplify dy ops
simplify dy ops


Former-commit-id: bf36c6641742d503529bb4e0bab21c2134ea34c0
clab/dynet,danielhers/dynet,shuheik/dynet,shuheik/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,xunzhang/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,chunyang-wen/dynet,xunzhang/dynet,clab/cnn,chunyang-wen/dynet,clab/cnn,xunzhang/dynet,shuheik/dynet,clab/cnn,shuheik/dynet,shuheik/dynet,chunyang-wen/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,clab/dynet,xunzhang/dynet,chunyang-wen/dynet,clab/dynet