## What changes were proposed in this pull request?

Currently collect_set/collect_list aggregation expression don't support partial aggregation. This patch is to enable partial aggregation for them.

## How was this patch tested?

Jenkins tests.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #16371 from viirya/collect-partial-support.