Add MLX handler for aten.flip

[mzl2233]

Summary:

• Add an MLX op handler for aten.flip by lowering flips to reverse slices along each requested dimension.
• Cover one-dimensional, multi-dimensional, and negative-dimension flip cases in the MLX op tests.

Validation:

• python3 -m py_compile backends/mlx/ops.py backends/mlx/test/test_ops.py

Fixes #18918

[pytorch-bot]

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[Copilot]

Pull request overview

Adds MLX delegate support for aten.flip by lowering flips into reverse SliceNode operations and adds op tests covering several flip dimension patterns.

Changes:

• Registers an MLX handler for torch.ops.aten.flip.default.
• Implements flip as chained reverse slices across requested dimensions.
• Adds MLX op-test coverage for 1-D, multi-dimensional, all-dimension, and negative-dimension flips.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
backends/mlx/ops.py	Adds the aten.flip MLX op handler using reverse slicing.
backends/mlx/test/test_ops.py	Adds FlipModel and FlipTest configurations for MLX op validation.

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