Port CPU decode_webp to stable ABI and retire the unstable image extension - #9557

Ports CPU webp decoder to stable ABI and retires the unstable image extension. Every image op now registers from image_stable.

Notes

• decode_webp is the last unstable image op.
• Unstable extension retirement: image.cpp/image.h and common.{h,cpp} are deleted, make_image_extension() and the _stable() source partition go away.
• ROCm: hipify's generated copy of the nvJPEG TU now links alongside the committed rocJPEG decoder, so the nvJPEG TU is guarded with #if !ROCJPEG_FOUND and the rocJPEG decoder registers its own schema.

Stable-ABI audit (torch-abi-audit)

Ran torch-abi-audit. The io/image migration is complete: image.so is gone, and image_stable.so audits as STABLE — 76 stable-shim symbols, 0 unstable.

    Package: torchvision
      Torch ABI:   UNSTABLE
      CPython ABI: n/a
      Bundled libs: 3
      -- bundled libs --
        [UNSTABLE] [not-abi3        ] _C.so            (stable_shim=0,  unstable=110)
        [STABLE  ] [not-abi3        ] _C_stable.so     (stable_shim=67, unstable=0)
        [STABLE  ] [not-abi3        ] image_stable.so  (stable_shim=76, unstable=0)