Add ParallelExecutor for training, validation and saving in Transformer

[guoshengCS]

Add ParallelExecutor for training, validation and saving in Transformer.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
0 out of 2 committers have signed the CLA.

❌ reyoung
❌ guoshengCS
_{You have signed the CLA already but the status is still pending? Let us recheck it.}