[SPARK-9533] [PYSPARK] [ML] Add missing methods in Word2Vec ML

After https://github.com/apache/spark/pull/7263 it is pretty straightforward to Python wrappers.

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #7930 from MechCoder/spark-9533 and squashes the following commits:

1bea394 [MechCoder] make getVectors a lazy val
5522756 [MechCoder] [SPARK-9533] [PySpark] [ML] Add missing methods in Word2Vec ML