Require method: custom for optuna/ax scheduler engines

[kmikowicz-wandb]

Summary

scheduler.engine: optuna and scheduler.engine: ax were rejected outright by config validation. They are now accepted, but only with method: custom; engine: wandb continues to require method: grid | random | bayes.

_SUPPORTED_METHODS_BY_ENGINE = {
    "wandb": ["grid", "bayes", "random"],
    "optuna": ["custom"],
    "ax": ["custom"],
}

The engine/method mismatch error now lists methods as `method: x` so it matches the wording of the other method: custom errors. Schema description for scheduler updated to document the mapping; tests updated to cover both directions for optuna/ax.

Link to Devin session: https://coreweave.devinenterprise.com/sessions/71a11719543544858580ce263664369a
Open in Devin Desktop: https://coreweave.devinenterprise.com/desktop/session/71a11719543544858580ce263664369a?variant=devin
Requested by: @kmikowicz-wandb

[devin-ai-integration]

Original prompt from Kaelan

SYSTEM:
<latest_message>
Kaelan Mikowicz (U0AGG225800) [ts=1788468606.980819]: In wandb/sweeps, ensure that for the config, scheduler: engine: ax and optuna require method: custom on the sweep, where engine: wandb requires a method: grid, random, bayes
</latest_message>

=== BEGIN THREAD HISTORY ===
Kaelan Mikowicz (U0AGG225800) [ts=1788468606.980819]: In wandb/sweeps, ensure that for the config, scheduler: engine: ax and optuna require method: custom on the sweep, where engine: wandb requires a method: grid, random, bayes
=== END THREAD HISTORY ===
Channel ID: D0BUDUPMV8U
Thread URL: https://weightsandbiases.slack.com/archives/D0BUDUPMV8U/p1788468606980819?thread_ts=1788468606.980819&amp;cid=D0BUDUPMV8U

The <latest_message> is the message that you should use to guide your goals + task for this session, and you should use the rest of the slack thread as context.
A [ts=...] marker on a Slack message is that message's timestamp. To act on a specific message with the slack tool (e.g. adding an emoji reaction via the reaction command), pass that value as timestamp along with the Channel ID — no extra lookup call is needed.

[devin-ai-integration]

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