Reinforce scheduling invariants (#17132)

Reinforce scheduling invariants (#17132)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17132

schedule() function is not supposed to throw exception and is supposed
to succeed in scheduling the full graph of tasks, potential errors (e.g. errors
from underlying thread pool, out of memory exceptions etc) are considered not
recoverable.
The invariant - the graph of tasks is either not executed or
executed in full before the call to finishRun()

Reviewed By: andrewwdye

Differential Revision: D14092457

fbshipit-source-id: a3e5d65dfee5ff5e5e71ec72bb9e576180019698