Add SeFi-Image pipeline

[jmliu206]

What does this PR do?

Adds Diffusers support for SeFi-Image, including:

• SeFiTransformer2DModel
• SeFiPipeline
• a conversion script for original SeFi checkpoints
• API docs and ToC entries
• fast model and pipeline tests

The pipeline supports loading converted SeFi checkpoints via from_pretrained, Qwen3-VL prompt encoding, SeFi dual-time denoising, classifier-free guidance for non-turbo checkpoints, turbo step validation, real checkpoint conversion, and model CPU offload.

Validation

PYTHONPATH=src python -m pytest tests/models/transformers/test_models_transformer_sefi.py tests/pipelines/sefi/test_pipeline_sefi.py -q
# 50 passed, 3 skipped, 6 warnings in 103.23s

PYTHONPATH=src ruff check docs/source/en/api/models/sefi_transformer2d.md docs/source/en/api/pipelines/sefi.md scripts/convert_sefi_to_diffusers.py src/diffusers/models/transformers/transformer_sefi.py src/diffusers/pipelines/sefi tests/models/transformers/test_models_transformer_sefi.py tests/pipelines/sefi/test_pipeline_sefi.py
# All checks passed!

PYTHONPATH=src ruff format --check scripts/convert_sefi_to_diffusers.py src/diffusers/models/transformers/transformer_sefi.py src/diffusers/pipelines/sefi tests/models/transformers/test_models_transformer_sefi.py tests/pipelines/sefi/test_pipeline_sefi.py
# 7 files already formatted

git diff --check
# passed

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• Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case.
  Add SeFi-Image pipeline #14086
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?
• Are you the author (or part of the team) of the model/pipeline (only applicable for model/pipeline related PRs)?

Who can review?

Potential reviewers for pipelines/models: @yiyixuxu @dg845 @asomoza