Add Stable Cascade segmented checkpointing support

[bghira]

Summary

Splits the Stable Cascade 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-sd3

Validation

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

[Copilot]

Pull request overview

Adds Stable Cascade (UNet) support for segmented/interval-based gradient checkpointing controls, aligning this model family with the broader segmented-checkpointing feature set introduced in the base stack branch.

Changes:

• Adds Stable Cascade UNet hooks to apply interval + segment-stride checkpoint selection via should_checkpoint_block.
• Extends safety-check allow-lists to permit interval/stride flags for stable_cascade.
• Adds/extends unit tests covering Stable Cascade checkpoint interval/stride behavior and model-family capability declarations.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
tests/test_stable_cascade_modules.py	Adds behavioral unit test verifying which Stable Cascade blocks are checkpointed under interval/stride settings.
tests/test_segmented_checkpointing_model_support.py	Registers Stable Cascade in the shared “model supports which controls” test matrix.
simpletuner/helpers/training/default_settings/safety_check.py	Adds stable_cascade to the interval + segment-stride supported model allow-lists.
simpletuner/helpers/models/stable_cascade/unet.py	Implements interval/stride-aware checkpoint dispatch in Stable Cascade UNet forward path.

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