Expose yaml file for a stage

[DachengLi1]

To integrate with other distributed systems with built in synchronization optimization, it will be great to transfer synchronization ability to it. It will be great if we can expose a yaml file generated by scheduler, so that the other system can use it.

Possible implementation: In Controller, write a new coroutine to detect whether this stage has finished all allocation, gain the lock of all coroutines, and expose necessary information.