[SPARK-3838][examples][mllib][python] Word2Vec example in python

This pull request refers to issue: https://issues.apache.org/jira/browse/SPARK-3838

Python example for word2vec
@mengxr 
it should look like the scala one, I think the following should be enough:
```
    from pyspark import SparkContext
    from pyspark.mllib.feature import Word2Vec

    sc = SparkContext(appName='Word2Vec')
    inp = sc.textFile("text8_lines").map(lambda row: row.split(" "))

    word2vec = Word2Vec()
    model = word2vec.fit(inp)

    synonyms = model.findSynonyms('china', 40)
    for word, cosine_distance in synonyms:
        print "{}: {}".format(word, cosine_distance)
```