[SPARK-3486][MLlib][PySpark] PySpark support for Word2Vec

@mengxr
Added PySpark support for Word2Vec
Change list
(1) PySpark support for Word2Vec
(2) SerDe support of string sequence both on python side and JVM side
(3) Test for SerDe of string sequence on JVM side
unpersist `data` after training explicitly because the user won't have access to it.