Add Qwen Image segmented checkpointing support

[bghira]

Summary

Splits the Qwen 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-pixart

Validation

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

[Copilot]

Pull request overview

Adds Qwen Image’s transformer to the segmented gradient checkpointing support surface so it can participate in the shared checkpoint-interval/segment-stride runtime introduced in the base PR stack.

Changes:

• Adds a Qwen Image coverage test ensuring the model exposes the expected segmented checkpointing control hooks.
• Extends safety-check allow-lists so qwen_image can use --gradient_checkpointing_interval and --gradient_checkpointing_segment_stride.
• Updates QwenImageTransformer2DModel to store/apply interval + segment stride and to use should_checkpoint_block(...) during the block loop (including recognizing unsloth* backends).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/test_segmented_checkpointing_model_support.py	Adds a Qwen Image capability coverage test for interval + segment stride hooks.
simpletuner/helpers/training/default_settings/safety_check.py	Allows qwen_image to use interval and segment-stride checkpointing flags.
simpletuner/helpers/models/qwen_image/transformer.py	Implements interval/segment-stride checkpoint decision logic and backend handling for Qwen Image blocks.

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