Port ps_roi_pool (MPS) to stable ABI. - #9618

Ports ps_roi_pool's MPS kernels to the stable ABI.

Notes

• Shader source moves to ps_roi_pool_metal_shader.h.
• The MPS backward accumulates with atomic_add, so it is nondeterministic and has to say so when torch.use_deterministic_algorithms(True) is set. A stable-ABI kernel cannot reach at::globalContext(), so the alert moves to the autograd wrapper in _autograd_registrations.py. Follows the roi_align port (Port roi_align (MPS) to stable ABI. #9612).

Stable-ABI audit (torch-abi-audit)

Ran torch-abi-audit. ps_roi_pool's MPS op moves into the fully-STABLE _C_stable.so and pulls in no new shim symbols. The unstable _C.so count is unchanged at 53: ps_roi_pool's MPS sources shared their unstable-API surface with the still-legacy kernels, so no symbol is uniquely removed until those migrate.

    Package: torchvision
      Torch ABI:   UNSTABLE
      CPython ABI: n/a
      Bundled libs: 3
      -- bundled libs --
        [UNSTABLE] [not-abi3] _C.so            (stable_shim=0,  unstable=53)
        [STABLE  ] [not-abi3] _C_stable.so     (stable_shim=78, unstable=0)
        [STABLE  ] [not-abi3] image_stable.so  (stable_shim=70, unstable=0)