Port CPU decode_png to stable ABI. - #9552

Ports CPU png decoder to stable ABI.

Notes:

Missing StableABI ops

• The EXIF orientation transform needs flip, which Stable-ABI does not expose yet. Added a stable_flip shim to common_stable.h following the same pattern as stable_permute from Port CPU png encoder to stable ABI. #9539.

Shared exif.h

• exif.h is shared with the unstable decode_jpeg, and its orientation transform returns tensors, so the header gains a stable-typed gated on the existing TORCH_TARGET_VERSION. The unstable branch is removed when decode_jpeg is ported.

Notes: libpng leaves the unstable extension

• decode_png was the last libpng consumer in the unstable image extension (encode_png moved in Port CPU png encoder to stable ABI. #9539), so libpng detection/linking now applies only to image_stable.

Stable-ABI audit (torch-abi-audit)

Ran torch-abi-audit against the built torchvision package. image_stable.so audits as STABLE 73 stable-shim symbols, 0 unstable The legacy image.so drops 45 → 37 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=37)
        [STABLE  ] [not-abi3        ] image_stable.so  (stable_shim=73, unstable=0)