[SPARK-16249][ML] Change visibility of Object ml.clustering.LDA to public for loading

## What changes were proposed in this pull request?
jira: https://issues.apache.org/jira/browse/SPARK-16249
Change visibility of Object ml.clustering.LDA to public for loading, thus users can invoke LDA.load("path").

## How was this patch tested?

existing ut and manually test for load ( saved with current code)

Author: Yuhao Yang <yuhao.yang@intel.com>
Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #13941 from hhbyyh/ldapublic.