Support negative dimensions on "aten.split_with_sizes_copy.default"

[jgibson2]

Summary

Support a negative dimension for MPS SplitWithSizes implementation.

Test plan

I re-registered this node visitor on a model that previously failed to export with the error message

RuntimeError: split_copy: dim -1 out of range for input tensor with 2 dimensions

and it succeeded.

[pytorch-bot]

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[Copilot]

Pull request overview

This PR adds support for negative dimension indexing in the aten.split_with_sizes_copy.default operator for the MPS backend. When a negative dimension is provided, it is converted to its positive equivalent by adding the input tensor's rank.

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[Copilot]

The validation condition on line 249 will always fail for negative dimensions after normalization. If the original dim was less than -len(input_shape), the normalized value will still be negative, but this is the intended behavior to catch out-of-bounds dimensions. However, the current implementation normalizes first, then validates, which means valid negative dimensions are handled correctly but the error condition dim < 0 is now checking the normalized value. The validation should only check dim >= len(input_shape) after normalization, or validate the original dimension range before normalization.

Suggested change

	if dim < 0 or dim >= len(input_shape):
	if dim >= len(input_shape):

[jgibson2]

The negative dimension could be outside the correct range even after normalization (e.g. if there are two dimensions and the input dimension is -3). This should probably still be an error.

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