[BUG] The pass "eliminate_nop_flatten" causes output mismatch

[BUG] The pass "eliminate_nop_flatten" causes output mismatch

Issue
Running the single pass eliminate_nop_flatten with onnxoptimizer 0.4.2 changes numerical outputs.

Environment

• Ubuntu 20.04
• Python 3.10
• onnx==1.19.0
• onnxruntime==1.23.2
• onnxoptimizer==0.4.2(latest)

Repro steps (run from this folder)

eliminate_nop_flatten_repro.tar.gz

1. Create a Python environment (Python 3.10) and install dependencies:

python3 -m venv .venv
source .venv/bin/activate
pip install -U pip
pip install -r requirements.txt

2. Optimize a case with only eliminate_nop_flatten (writes model.opt.onnx next to model.onnx):

• python optimize_model.py --case ./case_02084_seed36754215

3. Differential test original and optimized models with same oracle inputs (ORT graph optimization disabled):

• python diff_test.py --case ./case_02084_seed36754215

Expected
eliminate_nop_flatten should be semantics-preserving. Applying this pass alone should not change any output values.

Differential Test Output Details

1. case_03140_seed45116679 — mismatch; output[14] max_abs=4.022e+04 max_rel=4.022e+12, output[15] max_abs=5.190e+02 max_rel=5.120e+10; overall mismatch.
2. case_00324_seed22816775 — mismatch; output[3] max_abs=5.022e+18 max_rel=1.000e+00; overall mismatch.
3. case_09334_seed94166965 — mismatch; outputs 3, 4, 8-12, 14, 15 have max_abs=inf max_rel=inf (NaN-equal comparison enabled), output[13] max_abs=1.000e+00 max_rel=1.000e+00; overall mismatch.

Case: case_03140_seed45116679 (ORT graph optimizations disabled)
  output[14]: max_abs=4.022e+04, max_rel=4.022e+12, shape=(1, 21, 24)
  output[15]: max_abs=5.190e+02, max_rel=5.120e+10, shape=(32, 21, 24)
Overall: max_abs=4.022e+04, max_rel=4.022e+12

Case: case_00324_seed22816775 (ORT graph optimizations disabled)
  output[3]: max_abs=5.022e+18, max_rel=1.000e+00, shape=(51, 1, 1, 1)
Overall: max_abs=5.022e+18, max_rel=1.000e+00

Case: case_09334_seed94166965 (ORT graph optimizations disabled)
  outputs 3, 4, 8, 9, 10, 11, 12, 14, 15: max_abs=inf, max_rel=inf
  output[13]: max_abs=1.000e+00, max_rel=1.000e+00, shape=(4, 62)
Overall: max_abs=inf, max_rel=inf

Attachments

• requirements.txt
• optimize_model.py
• diff_test.py (runs original and optimized with same oracle inputs; ORT graph optimizations disabled)
• run_optimized_model.py (executes optimized model with same oracle inputs)
• case_03140_seed45116679/, case_04011_seed52014128/, case_00324_seed22816775/(original models, optimized models, and oracle inputs)

Start with the attached optimize_model.py and diff_test.py, using the listed case directories and the provided Python environment to reproduce the mismatch. Compare the original and optimized model outputs with ORT graph optimizations disabled; done means eliminate_nop_flatten preserves all reported outputs.