## What changes were proposed in this pull request?
Currently, `AFTAggregator` is not being merged correctly. For example, if there is any single empty partition in the data, this creates an `AFTAggregator` with zero total count which causes the exception below:
```
IllegalArgumentException: u'requirement failed: The number of instances should be greater than 0.0, but got 0.'
```
Please see [AFTSurvivalRegression.scala#L573-L575](6ecedf39b4/mllib/src/main/scala/org/apache/spark/ml/regression/AFTSurvivalRegression.scala (L573-L575)) as well.
Just to be clear, the python example `aft_survival_regression.py` seems using 5 rows. So, if there exist partitions more than 5, it throws the exception above since it contains empty partitions which results in an incorrectly merged `AFTAggregator`.
Executing `bin/spark-submit examples/src/main/python/ml/aft_survival_regression.py` on a machine with CPUs more than 5 is being failed because it creates tasks with some empty partitions with defualt configurations (AFAIK, it sets the parallelism level to the number of CPU cores).
## How was this patch tested?
An unit test in `AFTSurvivalRegressionSuite.scala` and manually tested by `bin/spark-submit examples/src/main/python/ml/aft_survival_regression.py`.
Author: hyukjinkwon <gurwls223@gmail.com>
Author: Hyukjin Kwon <gurwls223@gmail.com>
Closes#13619 from HyukjinKwon/SPARK-15892.
e3554605b3