[MLX] Add leaky_relu op handler

[devin-lai]

Summary:

• Add MLX lowering for aten.leaky_relu.default using existing GreaterEqual, Multiply, and Where nodes.
• Add focused MLX op tests for custom negative_slope values, including a slope above 1.

Test Plan:

• python -m py_compile backends/mlx/ops.py backends/mlx/test/test_ops.py
• git diff --check HEAD^..HEAD
• PATH="$PWD/.venv-mlx/bin:$PATH" .venv-mlx/bin/lintrunner backends/mlx/ops.py backends/mlx/test/test_ops.py
• .venv-mlx/bin/python -m executorch.backends.mlx.test.run_all_tests leaky_relu --timeout 180

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20305

• 📄 Preview Python docs built from this PR

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⚠️ 13 Awaiting Approval, 1 Pending

As of commit 58efda7 with merge base e28ef13 ():

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 WebGPU Backend (gh)
• Test XNNPACK Backend (gh)

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[devin-lai]

@pytorchbot label "release notes: apple"