Fix fuse_pad_into_conv incorrectly fusing when Pad contains negative values

[Copilot]

The fuse_pad_into_conv optimization would fuse a Pad node into Conv even when pads contained negative values (valid for Pad as crop operations, but invalid for Conv's pads attribute), producing an invalid model.

Changes

• _fuse_pad_into_conv.py: Added a guard in _FuseConvPadBase.check() to bail out when any computed pad value is negative, preventing fusion that would produce an invalid Conv node:

  if any(p < 0 for p in self._pads_list):
      return check_result.fail(f"{pads.name} must not contain negative values.")

• _fuse_pad_into_conv_test.py: Added a test case to test_unsupported_fuse_pad_into_conv verifying that fusion is rejected when spatial pads are negative.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>fuse_pad_into_conv optimization incorrectly fuses when 'pads' contain negative values</issue_title>
<issue_description>converting the attached ONNX model with the following script:

from onnxscript import ir
from onnxscript.optimizer import optimize

ir_model = ir.load("input.onnx")
optimized_model = optimize(ir_model)
ir.save(optimized_model, "output.onnx")

results in the following output model:

The Conv layer has negative pad values, which is not allowed per spec. This optimization should check for negative pad values.

input.onnx.zip

versions of relevant packages:

ml_dtypes         0.5.4
onnx              1.20.1
onnx-ir           0.1.16
onnxscript        0.6.2
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes fuse_pad_into_conv optimization incorrectly fuses when 'pads' contain negative values #2838

---

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[bas-aarts]

What is the reason the PR cannot be merged? It's a rathe simple fix.

[Copilot]

Pull request overview

This PR fixes an invalid optimization in the rewriter where Pad was being fused into Conv even when Pad.pads contained negative values (crop semantics), which would produce an invalid Conv node (negative pads are not allowed for Conv).

Changes:

• Add a guard in _FuseConvPadBase.check() to reject fusion when any computed pad value is negative.
• Add a regression test ensuring fusion is rejected when spatial pad values are negative.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
onnxscript/rewriter/rules/common/_fuse_pad_into_conv.py	Prevents Pad→Conv fusion when computed pads contain negative values, avoiding invalid Conv attributes.
onnxscript/rewriter/rules/common/_fuse_pad_into_conv_test.py	Adds coverage for the negative-padding (crop) case to ensure the fusion is correctly rejected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.64%. Comparing base (8fdb1e0) to head (94112d5).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2841   +/-   ##
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  Coverage   72.64%   72.64%           
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  Files         259      259           
  Lines       31652    31655    +3     
  Branches     2980     2981    +1     
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+ Hits        22994    22997    +3     
  Misses       7649     7649           
  Partials     1009     1009           

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