Port CPU png encoder to stable ABI.

[adabeyta]

Ports the CPU png encoder to stable ABI.

Notes: Missing StableABI stable_permute shim

• encode_png needs permute for its HWC↔CHW conversion, but Stable-ABI does not expose it yet. Followed torchcodec's stablePermute`.

Stable-ABI audit (torch-abi-audit)

Ran torch-abi-audit against the built torchvision package to verify the migrated extension stays on the stable surface. image_stable.so audits as STABLE — 67 stable-shim symbols, 0 unstable The legacy image.so / _C.so stay UNSTABLE (expected: their remaining ops still use the same private APIs), so the package-level verdict stays UNSTABLE until the rest move.

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=48)
    [STABLE  ] [not-abi3        ] image_stable.so  (stable_shim=67, unstable=0)

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/9539

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❌ 1 Unclassified Failure

As of commit 57bde81 with merge base 6104574 ():

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 !