Fix GemmTransposeFusion handling of identity Transpose - #32428

Fixes incorrect GemmTransposeFusion handling of identity transposes.

Changes:

• Restricts fusion to matrix transposes using [1, 0] or the rank-2 default permutation.
• Adds input-A regression coverage.

Suppressed comments (3)

onnxruntime/core/optimizer/gemm_transpose_fusion.cc:63

• The regression tests only place the Transpose on input A, so the newly guarded input-B path is not exercised. A future omission or regression of this IsMatrixTranspose check would still pass both new tests; add corresponding identity and [1, 0] cases with the Transpose feeding Gemm input 1 and assert transB.

  if (B_node_ptr != nullptr && B_node_ptr->OpType() == "Transpose" && IsMatrixTranspose(*B_node_ptr)) {

onnxruntime/core/optimizer/gemm_transpose_fusion.cc:84

• The output-Transpose permutation check is also new behavior, but the added tests cover only an input Transpose. Add output-side identity and [1, 0] cases so this path verifies that identity remains and a real matrix transpose still fuses with the expected swapped inputs/flags.

      output_node_ptr->OpType() == "Transpose" &&
      IsMatrixTranspose(*output_node_ptr)) {

onnxruntime/test/optimizer/gemm_transpose_fusion_test.cc:72

• These regressions exercise the new permutation check only for input A. The same predicate now gates input B and the output-Transpose rewrite, but neither path verifies that perm={0, 1} is preserved; a future omission or branch-specific regression there would still pass. Please add identity-permutation cases for B and the output path (the existing graph-transform tests cover their successful matrix-transpose fusion).

TEST(GemmTransposeFusionTest, IdentityInputTransposeIsNotFolded) {
  RunInputTransposeTest({0, 1}, {2, 3}, false, 0);
}

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced