fix(diffusers): drop compel from requirements to unblock pip resolver

[mudler]

compel 2.3.1 (latest, Nov 2025) declares transformers~=4.25 in its metadata, i.e. >=4.25,<5.0. After transformers 5.0 (2026-01-26) and huggingface-hub 1.0 (2025-10-27) shipped, the weekly DEPS_REFRESH cache rotation in CI started seeing the new majors and pip's resolver went into multi-hour backtracking storms walking every transformers 4.x candidate against every accelerate/hf-hub/tokenizers combination to find a set compel would accept. The 2026-04-29 backend-build for the diffusers backend (darwin-mps + l4t + cublas13-turboquant matrix cells) hit the GitHub Actions 6h job timeout still inside pip install — the build itself never started.

compel is the only hard upper bound on transformers in this stack (diffusers, accelerate, peft, optimum-quanto are all flexible), and upstream support for transformers 5 is still in flight: damian0815/ compel#129 ("Modernize Compel for Transformers 5") and #128 ("Bump transformers version to >5.0") are both open as of today.

backend.py only constructs Compel() when COMPEL=1 is set in the env (default off), so make compel a true optional extra:

• Wrap the top-level from compel import ... in try/except ImportError, mirroring the existing sd_embed pattern.
• Auto-disable COMPEL with a warning when the module isn't installed, instead of crashing on module load.
• Drop compel from all eight requirements-*.txt variants so the resolver no longer has to satisfy its transformers cap.
• Leave a TODO in backend.py and in each requirements file pointing at the upstream PR/issue, so the dependency can be reinstated once compel supports transformers >= 5.

Users who rely on weighted-prompt embeddings can opt in with a manual pip install compel alongside COMPEL=1; the warning emitted on startup tells them how.

Related upstream issues/PR:

• Modernize Compel for Transformers 5 and add diffusers smoke coverage damian0815/compel#129
• Bump transformers version to >5.0 damian0815/compel#128

Assisted-by: Claude:claude-opus-4-7 [Bash Read Edit WebFetch]