Add Boogu Image segmented checkpointing support

[bghira]

Summary

Splits the Boogu Image segmented checkpointing support model integration out of #2925.

• applies the model-family checkpointing hooks and support flags
• adds this family to the relevant safety-check allow-lists
• keeps the shared runtime/docs in the base PR so this diff stays model-specific

Stack

Base branch: agent/segmented-checkpointing-auraflow

Validation

• .venv/bin/python -m unittest tests.test_segmented_checkpointing_model_support -v
• .venv/bin/python -m unittest tests.test_boogu_image_model -v
• commit hooks: Black, isort, flake8, whitespace checks

[Copilot]

Pull request overview

Adds segmented gradient-checkpointing support for the Boogu Image transformer family by wiring interval/segment-stride knobs into the model and enabling the corresponding safety-check allow-lists, with targeted unit tests to validate the new controls and an autograd safety regression.

Changes:

• Add gradient_checkpointing_interval / gradient_checkpointing_segment_stride state + setters to BooguImageTransformer2DModel, and use should_checkpoint_block() to decide per-layer checkpointing.
• Allow-list boogu_image for interval + segmented stride in safety_check.
• Add unit tests covering checkpointing-control surface area and a regression test ensuring img_patch_embed_and_refine() is safe with custom autograd outputs.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/test_segmented_checkpointing_model_support.py	Adds a Boogu Image capability test to ensure interval/stride controls are exposed.
tests/test_boogu_image_model.py	Adds a regression test ensuring patch-embed/refine remains autograd-safe after layout writes.
simpletuner/helpers/training/default_settings/safety_check.py	Adds boogu_image to checkpointing interval + segment-stride allow-lists.
simpletuner/helpers/models/boogu_image/transformer.py	Implements interval/stride checkpointing decisions and adds cloning around embedder outputs.

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