cancellation_manager.pause_all_work() is insufficient to pause work

It is possible for work to continue running if no processes are registered on the cancellation manager when pause_all_work() is called.

Possible solutions:
Also send SIGSTOP to the worker python process- however this has a race condition: pause_all_work -> new work scheduled -> pause python -> work is still being done. pausing python first would prevent pause_all_work from running so that's not an option.

Alternatively, have cancellation manager send SIGSTOP upon process registration when paused. This should work, I think, and is probably the easiest solution.

to verify: add a time_sleep right under .pause_children() in train_locally and look at the cpu graph