Port CUDA jpeg decoder to stable ABI - #9533

Ports the CUDA jpeg decoder to stable ABI. The CUDA jpeg decoder lives in the image extension, so this also stands up image_stable (same dual-extension pattern to _C / _C_stable).

Structure: Image_stable Extension

Mirrors the _C / _C_stable split.

• torchvision.image: the existing extension, now built from all not-yet-ported image sources.
• torchvision.image_stable: the new extension, built only from migrated image sources (currently the jpeg decoder + its extension glue).

Layout Changes

• setup.py: splits the image sources between the two extensions. Migrated files added to STABLE_SOURCES, the existing _stable()/_not_stable() helpers route them into image_stable and keep everything else in image.

• make_image_stable_extension(): stable sibling of make_image_extension()).

• CMakeLists.txt: migrated files added to TORCHVISION_STABLE_SOURCES so the cmake build pins them identically.

• io/image.py: loads image_stable after image.

• Schema via STABLE_TORCH_LIBRARY_FRAGMENT(image), this extends the image namespace co-owned with the legacy .so.

• common_stable.h: torch-free ImageReadMode constants, so the stable TU doesn't pull full libtorch in via common.h.

• common_stable.cpp: extension-level glue for image_stable, the stable counterpart of common.cpp. Holds the Windows PyInit_image_stable stub extension glue, not tied to any op, it lives there instead of in a kernel file.

CUDA Stream/Event Model

• Decode runs on a dedicated stream, the caller's current stream then waits on it via the exact cudaEventCreate → Record → cudaStreamWaitEvent → Destroy sequence.
• syncStreams (extracted into a syncStreams helper). Current stream via aoti_torch_get_current_cuda_stream.

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 and never reaches into at:: / c10:: / torch::jit:: internals.

image_stable.so audits as STABLE — 65 stable-shim symbols, 0 unstable. The legacy image.so / _C.so stay UNSTABLE (expected: only nms and the jpeg decoder have
migrated), so the package-level verdict stays UNSTABLE until the rest move — same mid-migration shape as torchcodec.

 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=71)
     [STABLE  ] [not-abi3        ] image_stable.so  (stable_shim=65, unstable=0)