[SPARK-14298][ML][MLLIB] LDA should support disable checkpoint

## What changes were proposed in this pull request?
In the doc of [```checkpointInterval```](https://github.com/apache/spark/blob/master/mllib/src/main/scala/org/apache/spark/ml/param/shared/sharedParams.scala#L241), we told users that they can disable checkpoint by setting ```checkpointInterval = -1```. But we did not handle this situation for LDA actually, we should fix this bug.
## How was this patch tested?
Existing tests.

cc jkbradley

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #12089 from yanboliang/spark-14298.