[SPARK-6793] [MLLIB] OnlineLDAOptimizer LDA perplexity

Implements `logPerplexity` in `OnlineLDAOptimizer`. Also refactors inference code into companion object to enable future reuse (e.g. `predict` method).

Author: Feynman Liang <fliang@databricks.com>

Closes #7705 from feynmanliang/SPARK-6793-perplexity and squashes the following commits:

6da2c99 [Feynman Liang] Remove get* from LDAModel public API
8381da6 [Feynman Liang] Code review comments
17f7000 [Feynman Liang] Documentation typo fixes
2f452a4 [Feynman Liang] Remove auxillary DistributedLDAModel constructor
a275914 [Feynman Liang] Prevent empty counts calls to variationalInference
06d02d9 [Feynman Liang] Remove deprecated LocalLDAModel constructor
afecb46 [Feynman Liang] Fix regression bug in sstats accumulator
5a327a0 [Feynman Liang] Code review quick fixes
998c03e [Feynman Liang] Fix style
1cbb67d [Feynman Liang] Fix access modifier bug
4362daa [Feynman Liang] Organize imports
4f171f7 [Feynman Liang] Fix indendation
2f049ce [Feynman Liang] Fix failing save/load tests
7415e96 [Feynman Liang] Pick changes from big PR
11e7c33 [Feynman Liang] Merge remote-tracking branch 'apache/master' into SPARK-6793-perplexity
f8adc48 [Feynman Liang] Add logPerplexity, refactor variationalBound into a method
cd521d6 [Feynman Liang] Refactor methods into companion class
7f62a55 [Feynman Liang] --amend
c62cb1e [Feynman Liang] Outer product for stats, revert Range slicing
aead650 [Feynman Liang] Range slice, in-place update, reduce transposes