Discuss necessary info and access in barrier mode + YARN

In barrier mode, to run hybrid distributed DL training jobs, we need to provide users sufficient info and access so they can set up a hybrid distributed training job, e.g., using MPI.

This ticket limits the scope of discussion to Spark + YARN. There were some past attempts from the Hadoop community. So we should find someone with good knowledge to lead the discussion here.

Requirements:

• understand how to set up YARN to run MPI job as a YARN application
• figure out how to do it with Spark w/ Barrier