Add ZLab I1 segmented checkpointing support

[bghira]

Summary

Splits the ZLab I1 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-z-image

Validation

• .venv/bin/python -m unittest tests.test_segmented_checkpointing_model_support -v
• .venv/bin/python -m unittest tests.test_zlab_i1_model -v
• commit hooks: Black, isort, flake8, whitespace checks

[Copilot]

Pull request overview

Adds segmented checkpointing support for the ZLab I1 transformer family, integrating the shared segmented-checkpointing runtime hooks into the model implementation and ensuring the family is permitted by safety checks and covered by targeted tests.

Changes:

• Implement segmented checkpointing execution in ZlabI1Transformer2DModel.forward() using checkpoint_sequential_state and should_checkpoint_block.
• Add ZLab I1 to safety-check allow-lists for interval and segment-stride checkpointing controls.
• Add/extend unit tests validating both the presence of checkpointing controls and the ZLab I1 transformer’s use of the sequential-state helper.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
simpletuner/helpers/models/zlab_i1/transformer.py	Adds interval/stride fields + setters and integrates segmented checkpointing into the ZLab I1 forward pass.
simpletuner/helpers/training/default_settings/safety_check.py	Allow-lists zlab_i1 for checkpoint interval and segment stride options.
tests/test_zlab_i1_model.py	Adds a unit test asserting ZLab I1 segmented checkpointing routes through checkpoint_sequential_state.
tests/test_segmented_checkpointing_model_support.py	Adds model-family support test coverage for ZLab I1 checkpointing controls.

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