Add ERNIE segmented checkpointing support

[bghira]

Summary

Splits the ERNIE 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-cosmos3

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 ERNIE-family coverage for the segmented checkpointing feature set introduced in the stacked base branch, ensuring the ERNIE transformer exposes the expected control hooks and is permitted by safety checks.

Changes:

• Add ERNIE to segmented-checkpointing stride allow-lists in safety checks.
• Implement segmented-stride-aware checkpointing decision logic in the ERNIE transformer wrapper (via should_checkpoint_block) and expose the segment-stride setter.
• Expand the segmented checkpointing model-support test suite to include ERNIE.

Reviewed changes

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

File	Description
tests/test_segmented_checkpointing_model_support.py	Adds ERNIE to the model-family capability coverage tests (expects backend/interval/stride controls).
simpletuner/helpers/training/default_settings/safety_check.py	Allows ERNIE in the segment-stride supported model-family list.
simpletuner/helpers/models/ernie/transformer.py	Adds segment-stride setter and uses should_checkpoint_block for stride-aware checkpointing decisions; broadens Unsloth backend detection.
simpletuner/helpers/models/ernie/transformer_diffusers.py	Broadens Unsloth backend detection to match backend variants (e.g., unsloth-ffn).

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