Port CPU jpeg encoder to stable ABI.

[adabeyta]

Ports CPU jpeg encoder to stable ABI reusing image_stable extension.

Layout Changes

• setup.py: common_jpeg.cpp now builds into both extensions (unstable decode_jpeg still needs it).
• setup.py: make_image_stable_extension() gains libjpeg detection.

Stable-ABI audit (torch-abi-audit)

Ran torch-abi-audit against the built torchvision package.

image_stable.so audits as STABLE — 68 stable-shim symbols, 0 unstable The legacy image.so drops 48 → 45 unstable.

  Package: torchvision
    Torch ABI:   UNSTABLE
    CPython ABI: n/a
    Bundled libs: 4
    -- bundled libs --
      [UNSTABLE] [not-abi3        ] _C.so            (stable_shim=0,  unstable=110)
      [STABLE  ] [not-abi3        ] _C_stable.so     (stable_shim=67, unstable=0)
      [UNSTABLE] [uses-private-api] image.so         (stable_shim=0,  unstable=45)
      [STABLE  ] [not-abi3        ] image_stable.so  (stable_shim=68, unstable=0)

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/9543

• 📄 Preview Python docs built from this PR

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❌ 1 Unclassified Failure

As of commit ba76065 with merge base 0bc41e6 ():

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• .github/workflows/pr-labels.yml (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)

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[NicolasHug]

Thanks @adabeyta !