[AutoScheduler] Support early_stopping per task

[comaniac]

Per discussion in https://discuss.tvm.apache.org/t/ansor-task-scheduler-spending-too-much-trial-on-dead-end-tasks/8955, this PR improves the early stopping criteria in the task scheduler. Now we have two early stopping factors:

• TuningOptions.early_stopping: Like other options in this object, the early_stopping in TuningOptions affects each task.
• task_scheduler.tune(..., early_stopping=None): Since this is only for task_scheduler, this early_stopping applies to an entire tuning process.

Accordingly, when setting TuningOptions.early_stopping, a task will be stopped early if it has no improvement for N measurements, but the task scheduler will keep tuning rest tasks. On the other hand, task_scheduler.tune(..., early_stopping) serves as the original functionality that early stops an entire tuning process after no improvement for N measurements (regardless which tasks).

cc @merrymercy @masahi

[merrymercy]

I suggest that we can use TuneOption.early_stopping for all tasks, because TuneOption.num_measure_trials is the sum of measurements for all tasks.
We can use task_scheduler.tune(..., per_task_early_stopping=None) for per task early stopping.

[comaniac]

I suggest that we can use TuneOption.early_stopping for all tasks, because TuneOption.num_measure_trials is the sum of measurements for all tasks.
We can use task_scheduler.tune(..., per_task_early_stopping=None) for per task early stopping.

Fair enough. Updated.

[masahi]

@comaniac please kick the CI again

[masahi]

seems CI is having an issue

[masahi]

Thanks @comaniac @merrymercy