Preemptible instances support of `optuna tell`.

Motivation

When using optuna ask&tell on preemptible instances, optuna tell can raise RuntimeError on preemption because the trial can be already in one of finished state after preemption.

... # Some codes
optuna tell --storage $OPTUNA_STORAGE --study-name mystudy --trial-number 0 --values 1.0  # error in the second run
# <- preemption in the first run
... # Other codes

Description

Return a different exit code when the trial is already in a finished state so that users can distinguish errors caused by preemption and others (e.g., connection error to DB).

Alternatives

• Check the trial’s state and values first and avoid calling study.tell() when the requested (state, values) are identical to the current value of the trial.
  • Concern: Comparison of values can be problematic due to precision issues of numerical values.
• Guide users to write python script and use study.tell + try-catch on preemptible instances. (I’m currently using this.)

Additional context

I’m not quite sure a common way to handle this kind of situation and love to hear others’ voice.

[hvy]

It should be easy to implement, let me work on that.

[ytsmiling]

Thank you for creating the PR! Let me take a look.