[fix] missing doc-builder imports

[stevhliu]

#14131 removed the old container which provided sentencepiece which causes the doc builder to break now. This fixes it by guarding the import to fail at use time.

The other thing is to add bitsandbytes to https://github.com/huggingface/doc-builder/blob/main/src/doc_builder/mock_deps/diffusers.txt cc @mishig25

[mishig25]

Done in huggingface/doc-builder#805 — adds real:sentencepiece (installed for real by doc-builder light-install; kolors' tokenizer imports it at module level) and bitsandbytes as a mock to mock_deps/diffusers.txt. Validated the full 363-page diffusers docs build in a fresh venv with that registry. Since the shared workflows read the registry from the doc-builder@main checkout, the next main doc build here will pick it up as soon as #805 merges — no diffusers-side change needed for that part. Guarding the import as in this PR is still a good hardening on top.

[mishig25]

Update: after #805 (sentencepiece + bitsandbytes) the build progressed and then hit KeyError: 'optimum_quanto' on api/quantization.md. Two findings:

1. Mock quantization backends (optimum, torchao, gguf) for diffusers docs doc-builder#806 adds the quantization backends (optimum, torchao, gguf) as mocks — with DIFFUSERS_SLOW_IMPORT=yes the quantization page needs them importable, and mocking them means autodoc documents the real config classes rather than dummies. Verified the full 363-page build under exact CI conditions (DIFFUSERS_SLOW_IMPORT=yes, fresh venv, diffusers --no-deps).

2. The KeyError also exposed a latent diffusers bug the old container was masking: dummy_optimum_quanto_objects.py declares backend "optimum_quanto" but BACKENDS_MAPPING in utils/import_utils.py only has the key "quanto" — so when optimum-quanto is missing, requires_backends crashes with KeyError instead of raising the intended "requires optimum-quanto" ImportError. Worth a one-line fix here (add an "optimum_quanto" key or rename), independent of the doc build.

[HuggingFaceDocBuilderDev]

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[mishig25]

huggingface/doc-builder#806 is merged and the main doc build now passes ✅ — diffusers is fully on the light path (no container, --no-deps install, sentencepiece real, torch/bitsandbytes/optimum/torchao/gguf mocked), and the build populated the page cache for the runs that follow.

Nothing in the docs pipeline depends on this PR anymore, but the import guard and the optimum_quanto/quanto BACKENDS_MAPPING mismatch noted above are still worth landing on their own merit.

[DN6]

Thanks for flagging @mishig25 👍🏽 We're planning on removing quanto support, so that should resolve the issue.

[sayakpaul]

Do we still need this change given #14134 (comment)?