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fix(onnx): move a negative DequantizeLinear axis when transposing the weight - #2583

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SID-6921:fix/dq-transpose-negative-axis

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@SID-6921 SID-6921 commented Sep 29, 2026 •

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What does this PR do?

Type of change: Bug fix

transpose_dequantize_linear_weights() swaps the DequantizeLinear axis by matching the literal values 0 and 1:

if old_axis == 0:
    attr.i = 1
elif old_axis == 1:
    attr.i = 0

ONNX also allows a negative axis (Accepted range is [-r, r-1]). -1 and -2 match neither branch, so the weight is transposed while the axis keeps pointing at the dimension it had before. For a per-channel scale the axis attribute is the only thing tracking the channel dimension, so nothing else catches it.

Checked against NumPy on a [K, N] weight with a per-channel scale, for every legal 2D axis:

weight axis on main this PR
non-square 0, 1 correct correct
non-square -1, -2 converted model does not run correct
square -1, -2 runs, output wrong (max diff 4.09) correct

The square case is the one worth worrying about: the scale length still matches the axis it now points at, so nothing raises and the model simply computes something else.

Normalizing before the swap covers every legal 2D value, and the weight is already known to be 2D at that point:

attr.i = 1 - (old_axis % 2)

Testing

test_negative_axis_is_swapped, parametrized over 0, 1, -1, -2, asserts the resulting axis and compares onnxruntime output before and after the surgery. The two negative cases fail on main and pass here; the two non-negative cases pass on both, so the existing behaviour is unchanged.

One note on how that test runs onnxruntime. It sets ORT_DISABLE_ALL, because onnxruntime's graph optimizer does not treat a negative DequantizeLinear axis the same as its non-negative equivalent:

axis=1 vs axis=-1, same weights, before any surgery
optimizations disabled max diff 0.000000 (both match NumPy exactly)
optimizations enabled max diff 0.017256

With optimization on, the pre-surgery model is not a trustworthy baseline — I measured against it at first and it both masked the size of this bug and invented a residual that was not there. That looks like an onnxruntime issue rather than anything in this repo, so it is only worked around in the test, not addressed here. Worth knowing if you have other tests comparing quantized graphs across a rewrite.

tests/unit/onnx is 653 passed locally with 11 failures that also occur on pristine main in this environment (int4 paths needing cupy, three TestColumnMajorTransformation tests needing MSVC build tools). ruff check is unchanged.

Before your PR is "Ready for review"

  • Is this change backward compatible?: ✅ — axes 0 and 1 behave exactly as before
  • If you copied code from any other sources or added a new PIP dependency, did you follow guidance in CONTRIBUTING.md: N/A
  • Did you write any new necessary tests?: ✅
  • Did you update Changelog?: ✅
  • Did you get Claude approval on this PR?: N/A

Additional Information

Branched from main. Independent of #2553, #2554, #2567 and #2575 — no overlapping files.

Summary by CodeRabbit

  • Bug Fixes
    • Fixed an issue where transposing 2D column-major weights in quantized models could produce incorrect outputs when the quantization axis was negative or omitted. Axis handling now accounts for positive and negative values and the ONNX default, preserving model outputs after transposition.

… weight

transpose_dequantize_linear_weights swapped the axis attribute by matching the
literal values 0 and 1. ONNX also allows a negative axis, and -1 / -2 matched
neither, so the weight was transposed while the axis kept pointing at the
dimension it had before.

Checked against NumPy on a [K, N] weight, per-channel scale, for each legal 2D
axis. On a non-square weight the converted model no longer runs; on a square one
the scale length still matches, nothing complains, and the output is simply
wrong.

Normalize the axis before swapping it, which covers every legal 2D value.

The regression test runs onnxruntime with graph optimization disabled:
onnxruntime's optimizer does not treat a negative axis the same as its
non-negative equivalent, so an optimized session is not a trustworthy baseline
here.

Signed-off-by: Siddhardha Nanda <99672439+SID-6921@users.noreply.github.com>
@SID-6921
SID-6921 requested review from a team as code owners September 29, 2026 01:23
@SID-6921
SID-6921 requested review from vishalpandya1990 and a balanced review from Copilot September 29, 2026 01:23
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📒 Files selected for processing (2)
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📝 Walkthrough

The DequantizeLinear weight-transposition surgery now normalizes and swaps explicit axis values, including negative values. When the axis attribute is omitted, the surgery adds axis=0. Tests check axis mappings and compare model outputs before and after surgery.

Changes: DequantizeLinear axis handling

Layer / File(s) Summary
Update axis during weight transposition
modelopt/onnx/graph_surgery/dq_transpose.py, tests/unit/onnx/quantization/test_dq_transpose_surgery.py, CHANGELOG.rst
The surgery normalizes explicit axis values modulo 2 before swapping them. When the axis attribute is omitted, it adds axis=0. Tests cover explicit axes 0, 1, -1, and -2, and compare outputs with ONNX Runtime graph optimizations disabled. The changelog records the negative-axis fix.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Low

Merge Risk: ⚪ Minimal · up to aebc2

The axis-handling change has no identified merge-blocking issue; it is ready for normal checks.

Pre-merge checks: Passed 6
Passed Passed checks (6 passed)
Check name Status Explanation
Description Check Passed Check skipped - CodeRabbit’s high-level summary is enabled.
Title check Passed The title clearly and concisely describes the main change: correcting negative DequantizeLinear axis handling when transposing weights.
Docstring Coverage Passed Docstring coverage is 83.33% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 2 files. (1 skipped: 1 u…
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Out of Scope Changes check Passed Check skipped because no linked issues were found for this pull request.
Security Anti-Patterns Passed PASS. The pull request changes one Python file under modelopt and no Python files under examples. The added code only updates ONNX axis attributes. It does not add `torch.load(..., weights_only=Fa…
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@SID-6921

SID-6921 commented Oct 7, 2026

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Checking in — CodeRabbit approved and CI is green, open since late September with no human review yet. Happy to address anything needed.

@vishalpandya1990

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  • Should it also add handling for DQ node without axis parameter?

  • There appears to be code duplication between _build_dq_matmul_model() and _build_dq_matmul_model_with_axis() - can we add axis parameter to the former to handle new test cases?

@vishalpandya1990

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@hthadicherla for a quick look, viz.

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🧹 Nitpick comments (1)
tests/unit/onnx/quantization/test_dq_transpose_surgery.py (1)

177-178: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Use the existing model builder for each axis.

Both builders create the same graph, but one sets IR version 9 and the other sets IR version 10. Add an axis=0 parameter to _build_dq_matmul_model(). Size its scale and zero point from that axis, then use it in the parameterized test. This keeps the test models consistent and removes a second builder that must be maintained. As per path instructions, CONTRIBUTING.md says to “consolidate duplication when that makes the design simpler.”

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Review comment at @tests/unit/onnx/quantization/test_dq_transpose_surgery.py
around lines 177 - 178:
Update _build_dq_matmul_model to accept axis=0 and size its scale and zero point
according to that axis; use this builder for each axis in the parameterized
test. Remove _build_dq_matmul_model_with_axis so the tests share one model
builder and consistent IR version.

Source: Path instructions


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only still-valid issues, skip the rest with a brief reason, keep changes
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Nitpick comments:
Review comments at @tests/unit/onnx/quantization/test_dq_transpose_surgery.py:
- Around line 177-178: Update _build_dq_matmul_model to accept axis=0 and size
its scale and zero point according to that axis; use this builder for each axis
in the parameterized test. Remove _build_dq_matmul_model_with_axis so the tests
share one model builder and consistent IR version.

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_build_dq_matmul_model() and _build_dq_matmul_model_with_axis(axis) built
the same graph, differing only in whether axis was fixed at 0 or
parametrized, and in an accidental ir_version drift (9 vs 10). Add an
axis parameter (default 0) to the former and drop the latter, so both
TestDqTransposeSurgery and test_negative_axis_is_swapped share one
builder.

Signed-off-by: Siddhardha Nanda <99672439+SID-6921@users.noreply.github.com>
AAnoosheh added a commit that referenced this pull request Oct 9, 2026
### What does this PR do?

Type of change: Bug fix

`MFTLoss.forward` flattens both logit tensors to `(batch * positions,
vocab)` and its docstring only assumes the class dimension is last, so
leading dimensions are clearly meant to be allowed:

```python
soft_log_probs = soft_log_probs.view(-1, soft_log_probs.size(-1))   # (new B, C)
target_logits  = target_logits.view(-1, target_logits.size(-1))     # (new B, C)
soft_targets = self._prepare_corrected_distributions(target_logits, labels, ...)
```

The labels were passed through untouched, and
`_prepare_corrected_distributions` rejects anything that is not 1-D:

```
ValueError: Logits must be a 2D tensor and labels must be a 1D tensor.
```

So the shapes a language model actually produces — `(batch, seq_len,
vocab)` logits against `(batch, seq_len)` labels — cannot be used:

| logits | labels | on `main` | this PR |
|---|---|---|---|
| `(2, 8, 50)` | `(2, 8)` | `ValueError` | converges, matches the
flattened form exactly |
| `(16, 50)` | `(16,)` | works | unchanged |

That is the setting Minifinetuning (arXiv:2506.15702) is for, so in
practice a caller had to flatten the labels themselves and nothing
documented that.

Flattening them alongside the logits fixes it, and the docstring now
says what shape the labels are expected in.

### Testing

`test_mft_loss_accepts_sequence_shaped_logits` drives `MFTLoss` at `(2,
8, 50)` / `(2, 8)` and asserts the result equals the pre-flattened `(16,
50)` / `(16,)` call. It fails on `main` with the `ValueError` above and
passes here.

Worth noting why this was not caught: the existing
`test_distillation_model_mft` drives `MFTLoss` through a vision model,
whose logits are already `(batch, classes)` and labels already
`(batch,)`, so the flattening never does anything there.

`tests/unit/torch/distill` is 30 passed, 1 skipped locally. The skip is
`plugins/test_huggingface_kd.py`, which needs `transformers` and is not
installed in my environment; it exercises `LogitsDistillationLoss`,
which this change does not touch.

### Before your PR is "*Ready for review*"

- Is this change backward compatible?: ✅ — already-1-D labels reshape to
themselves, so existing callers are unaffected
- If you copied code from any other sources or added a new PIP
dependency, did you follow guidance in `CONTRIBUTING.md`: N/A
- Did you write any new necessary tests?: ✅
- Did you update
[Changelog](https://github.com/NVIDIA/Model-Optimizer/blob/main/CHANGELOG.rst)?:
✅
- Did you get Claude approval on this PR?: N/A

### Additional Information

Unrelated to my open ONNX PRs (#2553, #2554, #2567, #2575, #2583) — no
shared files.

One thing I noticed next door and did not touch, in case it is of
interest: `LogitsDistillationLoss` defaults to `reduction="mean"`, while
`MFTLoss` in the same file defaults to `"batchmean"`. PyTorch warns on
every call that `"mean"` is not the KL divergence value and that it will
be changed to behave as `"batchmean"` in a future major release, so the
default path is currently a factor of the vocabulary size away from the
other two reductions and will shift silently when that lands. Changing a
training default is your call rather than mine, so I have left it alone
— happy to open a separate issue if useful.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Improved handling of sequence-shaped labels when calculating
distillation loss, ensuring results match equivalent flattened inputs.
* **Tests**
* Added coverage verifying consistent loss results for sequence-shaped
and flattened inputs.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Siddhardha Nanda <99672439+SID-6921@users.noreply.github.com>
Co-authored-by: Asha Anoosheh <aanoosheh@nvidia.com>
@vishalpandya1990

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It seems DQ node with default axis params remain to be handled. Suggest to fix this as well, while at it.

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Good catch. The axis attribute is optional and defaults to 1 per the ONNX spec, and the old loop only acted when it found an explicit axis attr, so a node that relied on the default never got corrected. Pushed a fix: if no axis attribute is found, one is added with the post-transpose value. Added a test that builds a DQ node with no axis attribute at all and checks both the resulting attribute and the actual output match the pre-surgery model.

@SID-6921
SID-6921 force-pushed the fix/dq-transpose-negative-axis branch from aebc294 to 021b82a Compare October 13, 2026 00:01
Comment on lines +317 to +320
if not axis_attr_found:
dq_node.attribute.append(helper.make_attribute("axis", 0))
if verbose:
logger.debug(" Added axis: 1 (default) -> 0")

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This change should be fine for ONNX models with opset > 12.

But for older models with opsets from 10-12 this would be an issue because DequantizeLinear doesn't have an axis attribute for those opsets, so this code would cause an error. Older code would not have regressed for such models because we were not forcing an axis attribute if it didn't exist.

IMO, we should keep it axis free for opset < 13 and add a default axis only when dealing with higher opset nodes.

@vishalpandya1990 any thoughts on this ?

@vishalpandya1990 vishalpandya1990 Oct 13, 2026 •

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Yes, this case should be for opset 13+ only: add axis only when the scale has more than one element (i.e. it's not a per-tensor DQ; opset < 13 only supports per-tensor), or alternatively only when the model's default-domain opset is ≥ 13.

…tive-axis

Signed-off-by: Siddhardha Nanda <99672439+SID-6921@users.noreply.github.com>

# Conflicts:
#	CHANGELOG.rst

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