unify usage of key for populate parameterCollection in cpp end
unify usage of key for populate parameterCollection in cpp end


Former-commit-id: c463e1801196b30a327a12b3cf0acfcfc5c3556c
xunzhang/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,chunyang-wen/dynet,chunyang-wen/dynet,clab/dynet,clab/cnn,danielhers/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,xunzhang/dynet,danielhers/dynet,clab/dynet,clab/dynet,chunyang-wen/dynet,xunzhang/dynet,clab/cnn,chunyang-wen/dynet,shuheik/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,clab/cnn,shuheik/dynet,xunzhang/dynet