## What changes were proposed in this pull request?

Python API supports mode save/load in FPGrowth
JIRA: [https://issues.apache.org/jira/browse/SPARK-7106](https://issues.apache.org/jira/browse/SPARK-7106)
## How was the this patch tested?

The patch is tested with Python doctest.

Author: Kai Jiang <jiangkai@gmail.com>

Closes #11321 from vectorijk/spark-7106.