fix pycnn cg bug
fix pycnn cg bug


Former-commit-id: 51b3590567a7c42759dbfadfb3e65da2d82d9762
shuheik/dynet,clab/dynet,danielhers/dynet,chunyang-wen/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,xunzhang/dynet,clab/dynet,xunzhang/dynet,clab/cnn,xunzhang/dynet,shuheik/dynet,danielhers/dynet,chunyang-wen/dynet,clab/cnn,shuheik/dynet,danielhers/dynet,xunzhang/dynet,danielhers/dynet,danielhers/dynet,clab/dynet,chunyang-wen/dynet,danielhers/dynet,clab/cnn,chunyang-wen/dynet,clab/dynet,clab/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet