What does someone say who leaves the imam during the taraweeh prayer without praying witr in order to complete it later?

Praying at home? No, the sunnah is to join the imam, so when the imam finishes the witr prayer, he completes the rak'ah.

After that, then he prays, then he prays at home. So if he prayed witr with him, for example, and finished with him, and wanted to pray, he can pray after that and his first witr is sufficient. May Allah reward you with goodness, and the du'a reported from the Messenger ﷺ.