Add Sana segmented checkpointing support

[bghira]

Summary

Splits the Sana 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-qwen-image

Validation

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

[Copilot]

🟢 Ready to approve

Changes are small, consistent with existing model-family patterns, and include a targeted unit test plus the required safety-check allow-list update.

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Pull request overview

Adds Sana model-family support for segmented gradient checkpointing controls by wiring in the shared checkpointing helper logic, enabling the segment-stride knob, and ensuring the model is permitted by the safety-check allow-lists and covered by the model-support test suite.

Changes:

• Add a Sana-specific segmented checkpointing support test to validate the expected control surface exists.
• Allow Sana in gradient_checkpointing_segment_stride_supported_models so the CLI/env/WebUI setting can be used without being rejected.
• Update SanaTransformer2DModel to store/apply gradient_checkpointing_segment_stride and to use should_checkpoint_block(...) for consistent interval/stride checkpoint decisions; broaden Unsloth backend detection to startswith("unsloth").

File summaries

File	Description
tests/test_segmented_checkpointing_model_support.py	Adds a Sana model-family support test entry for segmented checkpointing controls.
simpletuner/helpers/training/default_settings/safety_check.py	Adds sana to the segment-stride gradient checkpointing allow-list.
simpletuner/helpers/models/sana/transformer.py	Implements segment-stride state + setter and uses shared checkpointing decision helper; updates Unsloth backend check.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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