[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 apache#12089 from yanboliang/spark-14298.

mllib/src/main/scala/org/apache/spark/mllib/impl/PeriodicCheckpointer.scala

@@ -52,7 +52,8 @@ import org.apache.spark.storage.StorageLevel
  *  - This class removes checkpoint files once later Datasets have been checkpointed.
  *    However, references to the older Datasets will still return isCheckpointed = true.
  *
- * @param checkpointInterval  Datasets will be checkpointed at this interval
+ * @param checkpointInterval  Datasets will be checkpointed at this interval.
+ *                            If this interval was set as -1, then checkpointing will be disabled.
  * @param sc  SparkContext for the Datasets given to this checkpointer
  * @tparam T  Dataset type, such as RDD[Double]
  */
@@ -89,7 +90,8 @@ private[mllib] abstract class PeriodicCheckpointer[T](
     updateCount += 1
 
     // Handle checkpointing (after persisting)
-    if ((updateCount % checkpointInterval) == 0 && sc.getCheckpointDir.nonEmpty) {
+    if (checkpointInterval != -1 && (updateCount % checkpointInterval) == 0
+      && sc.getCheckpointDir.nonEmpty) {
       // Add new checkpoint before removing old checkpoints.
       checkpoint(newData)
       checkpointQueue.enqueue(newData)

mllib/src/main/scala/org/apache/spark/mllib/impl/PeriodicGraphCheckpointer.scala

@@ -69,7 +69,8 @@ import org.apache.spark.storage.StorageLevel
  *  // checkpointed: graph4
  * }}}
  *
- * @param checkpointInterval Graphs will be checkpointed at this interval
+ * @param checkpointInterval Graphs will be checkpointed at this interval.
+ *                           If this interval was set as -1, then checkpointing will be disabled.
  * @tparam VD  Vertex descriptor type
  * @tparam ED  Edge descriptor type
  *