feat(mlx): add handler for aten.roll

[IshanG97]

Summary

Adds an MLX delegate handler for aten.roll, mapping torch.roll onto mlx::core::roll via a new RollNode in the schema. Replaces the default decomposition (index_select + arange + cat) with a single native kernel — needed by Swin Transformer's shift-window attention.

Flat roll (dims=[]) raises NotImplementedError for now; no known consumer needs it yet.

Generated files (MLXLoader.*, schema_generated.h, mlx_graph_schema.py, _generated_serializers.py, _generated_inspector.py, _generated/) are regenerated from schema.fbs by backends/mlx/CMakeLists.txt at build time and are deliberately not committed.

Fixes #18919.

Test plan

• python backends/mlx/serialization/generate.py — regenerates cleanly with RollNode in all expected outputs.
• lintrunner --skip MYPY --paths-cmd 'git diff --name-only upstream/main' — no issues.
• End-to-end run_all_tests -k roll not run locally (no executorch build on this machine); relying on CI. Happy to push fixes if it finds anything.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19038

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⚠️ 12 Awaiting Approval

As of commit b2f3bf0 with merge base 48ec3fc ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

• Build documentation (gh)
• Build Presets (gh)
• Cadence Build & Test (gh)
• Lint (gh)
• MLX (gh)
• periodic (gh)
• pull (gh)
• Test ARM Backend (gh)
• Test CoreML Backend (gh)
• Test QNN Backend (gh)
• Test Vulkan Backend (gh)
• Test XNNPACK Backend (gh)

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