Deprecate keep_end truncation mode

[albertvillanova]

Deprecate "keep_end" truncation mode.

See related discussion:

• Remove truncation_mode from DPO #5372

This PR deprecates the "keep_end" truncation mode in the DPOConfig and SFTConfig classes, warning users that it will be removed in version 2.0.0. The changes update documentation, add a deprecation warning at runtime, and adjust tests to expect this warning.

Changes

Deprecation of "keep_end" truncation mode:

• Updated the docstring and help metadata for the truncation_mode parameter in DPOConfig and SFTConfig to state that only "keep_start" is supported, and that "keep_end" is deprecated and will be removed in v2.0.0.
• Added a runtime warning in __post_init__ to emit a FutureWarning if "keep_end" is used, guiding users to switch to "keep_start".

Test updates:

• Modified the regression tests test_train_vlm_keep_end_raises and test_skip_prepare_dataset_passes_truncation_to_text_collator to expect a FutureWarning when "keep_end" is used, ensuring the deprecation warning is tested.

---

Note

Medium Risk
Behavior is backwards-compatible but introduces a new FutureWarning that can fail user pipelines configured to treat warnings as errors; otherwise the change is localized to config initialization and tests.

Overview
DPOConfig now deprecates truncation_mode="keep_end": docs/help text were updated to state keep_start is the only supported mode, and using keep_end emits a FutureWarning indicating it will be removed in v2.0.0.

Tests were updated to assert the new deprecation warning is raised when initializing DPOConfig with keep_end (while still expecting the existing VLM ValueError guard).

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