mirrored_strategy_runner.py  for multiple executors / workers

Hi, could I ask if there is a way to get the spark_tensorflow_distributor [MirroredStrategyRunner](https://github.com/tensorflow/ecosystem/blob/master/spark/spark-tensorflow-distributor/spark_tensorflow_distributor/mirrored_strategy_runner.py) to run on multiple executors/workers? Currently the code looks like it only supports multiple slots.