Add ONNX GraphSurgeon transformer block surgery example

[KOKOSde]

Summary

• add a runnable ONNX GraphSurgeon example for conservative transformer-block graph cleanup
• generate a small transformer-like ONNX graph with an Identity and canceling Transpose pair
• show how to remove those no-op structures while keeping the graph in standard ONNX operators
• register the example in the existing ONNX GraphSurgeon example test list

Validation

• python3 -m py_compile tools/onnx-graphsurgeon/examples/13_transformer_block_surgery/generate.py tools/onnx-graphsurgeon/examples/13_transformer_block_surgery/surgeon.py tools/onnx-graphsurgeon/tests/test_examples.py
• git diff --check
• In a temporary virtualenv with numpy/onnx/onnxruntime/colored installed: generated model.onnx, ran surgeon.py, checked both ONNX models, and ran CPU inference on both outputs

Note: pytest tools/onnx-graphsurgeon/tests/test_examples.py -k 13_transformer_block_surgery could not be run directly in this local checkout because the test harness invokes bare python3 and the system Python is externally managed without ONNX installed. The README commands were validated manually in a virtualenv using the local ONNX GraphSurgeon source via PYTHONPATH.