Rewrite ORTModule background task coordination (#6700)
* Introduce OrtTasks to replace EventPool
* return run_id to frontend
* pass run_id to backward
* OrtTasks support multiple bg_events
* make message_queue a member of orttask
* Replace MessageQueue with std::promise
* Move status_promise into Task
* Move terminate flag into Task
* Reenable previously disabled UTs
* Add unit tests
* Replace condition variables with std::promise
* Move to CreateBackgroundTask in the main thread
* return status and output in forward_future
* use throw for terminating background thread
* cleanup tasks at destructor
* reenable test_mixed_nnmodule_ortmodules_training
* add mutex for ORTTasks functions
* add mutex for bg_threads
* delay tests before start
* add ut for multi-task common backbone
Co-authored-by: Sherlock Huang <bahuang@OrtTrainingDev3.af05slrtruoetgaxwwjv5nsq5e.px.internal.cloudapp.net>
