feat(onnxslim_pass):Add split_to_slice pass test and update qkv_split…

[co-seven]

… pass

[Copilot]

Pull request overview

This PR extends xslim.onnxslim_pass with two new ONNX graph rewrites: (1) a Split -> Slice rewrite when only a single split output is actually used, and (2) an additional QKV MatMul fusion pattern that recognizes a MatMul -> Add -> Reshape -> Transpose -> Split -> Squeeze layout and rewrites it into three independent Q/K/V branches. It also adds/updates unit tests to validate the new fusion and rewrite behaviors.

Changes:

• Add a new fusion pass (split_to_slice) that rewrites eligible Split nodes into a single Slice.
• Add a new QKV fusion matcher for MatMul/Add/Reshape/Transpose/Split/Squeeze patterns (contiguous Q/K/V weight layout).
• Add tests covering the new QKV fusion and the Split -> Slice rewrite/keep behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/test_onnxslim_pass.py	Adds models + assertions to validate the new QKV fusion pattern and Split -> Slice rewrite behavior.
src/xslim/onnxslim_pass/split_to_slice.py	Introduces the SingleConsumerSplitToSlicePatternMatcher and rewrite logic.
src/xslim/onnxslim_pass/qkv_split.py	Adds a new matcher that fuses the reshape/transpose/split/squeeze QKV pattern into 3 branches.
src/xslim/onnxslim_pass/__init__.py	Ensures the new split_to_slice pass is imported/registered.

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