fix: prepare_data crashes with non shuffle split + sample_weight (#887, #1553)

[immu4989]

Why are these changes needed?

Two related bugs in GenericTask.prepare_data that surface together whenever a user combines a non-shuffle split (split_type="time" or "group") with sample_weight:

1. [Bug]: holdout + sample_weight + time/group split raises ValueError due to missing elif in prepare_data branching #1553 (ValueError: Found input variables with inconsistent numbers of samples) — prepare_data was splitting the data twice. The if split_type == "group": branch at flaml/automl/task/generic_task.py:956 was a standalone if, so after the time/group branch ran (correctly subsetting state.fit_kwargs["sample_weight"]), the chained elif self.is_classification() / elif self.is_regression() branches also fired and tried to split the already-subset weight against the still-full X_train_all. One-character fix: chain the group branch off the preceding time branch with elif.

2. 'AutoMLState' has no attribute 'sample_weight_all' when split_type='time' #887 (AttributeError: 'AutoMLState' has no attribute 'sample_weight_all') — AutoMLState.prepare_sample_train_data reads self.sample_weight_all whenever "sample_weight" is in fit_kwargs, but the attribute was only set inside the SHUFFLE_SPLIT_TYPES branch. For split_type="time" or "group" (or any other non-shuffle path), the attribute was never created, raising the AttributeError during the final retrain step. Fix proposed by @sonichi in 'AutoMLState' has no attribute 'sample_weight_all' when split_type='time' #887 (2023): initialize state.sample_weight_all = sample_weight_full once before the shuffle branching; the SHUFFLE branch still overwrites it after shuffle(...) returns the shuffled weight.

The two bugs surface in the same call (AutoML.fit(... split_type="time", sample_weight=..., retrain_full=True)) — the ValueError fires first in prepare_data, and once that's fixed, the AttributeError becomes reachable during the retrain step. So both fixes share a single regression test.

Related issue number

Closes #887
Closes #1553

Checks

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[Copilot]

Pull request overview

Fixes two related bugs in GenericTask.prepare_data that crash AutoML.fit when combining a non-shuffle split (time/group) with sample_weight. The first bug was caused by a missing elif, leading to double-splitting of the weights; the second by state.sample_weight_all only being set on the shuffle path, causing an AttributeError during final retraining.

Changes:

• Chain the group holdout branch as elif of the time branch so the time/group/classification/regression split branches are mutually exclusive.
• Initialize state.sample_weight_all = sample_weight_full before split branching so non-shuffle paths populate the attribute consumed by AutoMLState.prepare_sample_train_data.
• Add a regression test exercising split_type="time" + sample_weight + retrain_full=True holdout.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
flaml/automl/task/generic_task.py	Initializes sample_weight_all for all split paths and makes the group holdout branch mutually exclusive with the surrounding branches.
test/automl/test_split.py	Adds regression test covering issues #887 and #1553 (time split + sample_weight + retrain_full).

[thinkall]

LGTM. Thank you very much for the PR, @immu4989 .