Fix build errors of GridSample and test failures in test_attention_fusion.py

[tianleiwu]

Description

This PR addresses a build error and subsequent test failures related to recent changes in GridSample and the transformer optimizer. Related PRs: #27201, #27556.

Changes

1. Fix GridSample Build Error

• Removed an unused local variable mode_str in onnxruntime/core/providers/cuda/tensor/grid_sample.cc that was causing a warning (treated as error). After enabling c++20 in recent comment, this warning surfaces.
• Ref: grid_sample.cc

2. Update GridSample Tests

• Updated onnxruntime/test/providers/cpu/tensor/grid_sample_test_custom.inc to use default execution providers in RunTests instead of a hardcoded opset version, ensuring compatibility across different environments.

3. Revert Transformer Fusion Fallback

• Reverted a recent change in onnxruntime/python/tools/transformers/fusion_skiplayernorm.py that enabled a fallback for SkipLayerNormalization fusion when symbolic shape inference fails.
• This revert was necessary to avoid regressions in GPT-2 tests where model definitions contain typos that intentionally (or coincidentally) break shape inference.
• Ref: fusion_skiplayernorm.py

4. Restore Transformer Test Parity

• Updated onnxruntime/test/python/transformers/test_attention_fusion.py specifically test_qwen3_normalization_fusion to match the expected node counts after reverting the fusion fallback.
• Ref: test_attention_fusion.py

Verification

• build completed successfully.
• onnxruntime/test/python/transformers/test_attention_fusion.py passes with "OK".

[Copilot]

Pull request overview

This PR fixes a build error in the CUDA GridSample operator and reverts a transformer fusion fallback that caused test regressions, following up on PRs #27201 and #27556.

Changes:

• Removed an unused local variable mode_str in the GridSample CUDA constructor that shadowed per-branch locals and caused a build warning/error.
• Reverted the SkipLayerNormalization fusion fallback in fusion_skiplayernorm.py (from PR #27556) that broke GPT-2 tests, restoring the early-return when symbolic shape inference fails.
• Updated GridSample custom tests and Qwen3 attention fusion test expectations to match the fixed code.

Reviewed changes

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

File	Description
onnxruntime/core/providers/cuda/tensor/grid_sample.cc	Remove unused outer-scope mode_str variable to fix shadowing build error
onnxruntime/test/providers/cpu/tensor/grid_sample_test_custom.inc	Use default GetExecutionProviders() matching convention of all other GridSample tests
onnxruntime/python/tools/transformers/fusion_skiplayernorm.py	Revert fallback: early-return when shape inference fails instead of using default skip_index
onnxruntime/test/python/transformers/test_attention_fusion.py	Update Qwen3 test expectations (4 SLN, 0 SSLN) to match reverted fusion behavior

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