Support word2vec

Support Distributed Representation, word2vec.

• Original paper http://web2.cs.colmbia.edu/~blei/seminar/2016_discrete_data/readings/MikolovSutskeverChenCorradoDean2013.pdf
• word2vec Explained https://arxiv.org/pdf/1402.3722.pdf
• Network-Efficient Distributed Word2vec Training System for Large Vocabularies https://arxiv.org/abs/1606.08495
• Parallelizing Word2Vec in Shared and Distributed Memory https://pdfs.semanticscholar.org/cced/c38f68ffaf51cf8c31cd6c6b5c2cf033f91a.pdf

Implementations

• https://spark.apache.org/docs/latest/mllib-feature-extraction.html#word2vec
  https://github.com/apache/spark/pull/1719
• https://github.com/deeplearning4j/deeplearning4j/blob/master/deeplearning4j-scaleout/spark/dl4j-spark-nlp/src/main/java/org/deeplearning4j/spark/models/embeddings/word2vec/Word2Vec.java