[SPARK-5567] [MLLIB] Add predict method to LocalLDAModel

jkbradley hhbyyh

Adds `topicDistributions` to LocalLDAModel. Please review after #7757 is merged.

Author: Feynman Liang <fliang@databricks.com>

Closes #7760 from feynmanliang/SPARK-5567-predict-in-LDA and squashes the following commits:

0ad1134 [Feynman Liang] Remove println
27b3877 [Feynman Liang] Code review fixes
6bfb87c [Feynman Liang] Remove extra newline
476f788 [Feynman Liang] Fix checks and doc for variationalInference
061780c [Feynman Liang] Code review cleanup
3be2947 [Feynman Liang] Rename topicDistribution -> topicDistributions
2a821a6 [Feynman Liang] Add predict methods to LocalLDAModel