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Fix Diffusers export dtype resolution - #2225

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Fix Diffusers export dtype resolution#2225
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asma/fix_diffusers_export_dtype

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@realAsma realAsma commented Aug 20, 2026

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What does this PR do?

Type of change: Bug fix

When an explicit export dtype is provided, tolerate model configs that do not define torch_dtype. This fixes Diffusers export under dependency combinations where the pipeline config is a FrozenDict without that field, while preserving existing mismatch warnings when a configured dtype is available.

Usage

N/A — no API change.

Testing

  • pytest_pwd tests/unit/torch/export/test_unified_export_hf.py -k resolve_export_dtype (1 passed)
  • pytest_pwd tests/unit/torch/export/test_unified_export_hf.py (23 passed)
  • pre-commit run --files modelopt/torch/export/unified_export_hf.py tests/unit/torch/export/test_unified_export_hf.py

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  • Is this change backward compatible?: ✅
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  • Did you write any new necessary tests?: ✅
  • Did you update Changelog?: N/A — this is a focused compatibility fix.
  • Did you get Claude approval on this PR?: N/A — draft PR.

Additional Information

This is independent of PR #2223 and addresses the unrelated minimum-Transformers Diffusers export failure.

Summary by CodeRabbit

  • Bug Fixes

    • Improved export handling for models without a configured data type.
    • Export now reliably uses the requested type, the model’s weight type, or a safe default.
    • Prevented unnecessary data type mismatch warnings when no model type is configured.
  • Tests

    • Added regression coverage for exports with unspecified and explicitly requested data types.

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📝 Walkthrough

Walkthrough

The export dtype resolver now handles models without config.torch_dtype. It uses the configured dtype, the first parameter dtype, or torch.float16. Tests cover configured, fallback, matching, mismatched, and empty Diffusers configurations.

Changes

Export dtype fallback

Layer / File(s) Summary
Dtype resolution and regression coverage
modelopt/torch/export/unified_export_hf.py, tests/unit/torch/export/test_unified_export_hf.py
_resolve_export_dtype safely handles missing configured dtypes. Tests cover fallback behavior, dtype matching and mismatches, warning handling, and empty Diffusers configuration fallback to torch.float32.

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

Merge Risk: 🔵 Low · up to 22a10

The export dtype resolution change is localized and preserves configured-dtype mismatch warnings, but the regression suite does not yet exercise the exact FrozenDict-without-torch_dtype case. The PR is mergeable with explicit follow-up to add that targeted test.

Suggested reviewers: sugunav14

🚥 Pre-merge checks | ✅ 5 | ❌ 1

❌ Failed checks (1 warning)

Check name Status Explanation Resolution
Docstring Coverage ⚠️ Warning Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 2 files. Write docstrings for the functions missing them to satisfy the coverage threshold.
✅ Passed checks (5 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: fixing Diffusers export dtype resolution.
Linked Issues check ✅ Passed Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check ✅ Passed Check skipped because no linked issues were found for this pull request.
Security Anti-Patterns ✅ Passed The PR changes only dtype resolution and tests; the cumulative added lines contain no listed unsafe loading, eval/exec, # nosec, trust_remote_code, or dependency additions.
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  • Commit unit tests in branch asma/fix_diffusers_export_dtype

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.41%. Comparing base (a2fbac7) to head (22a10db).

Additional details and impacted files
@@            Coverage Diff             @@
##             main    #2225      +/-   ##
==========================================
- Coverage   78.99%   78.41%   -0.59%     
==========================================
  Files         522      522              
  Lines       60599    60603       +4     
==========================================
- Hits        47872    47521     -351     
- Misses      12727    13082     +355     
Flag Coverage Δ
examples-diffusers 20.70% <0.00%> (-0.01%) ⬇️
examples-gpt-oss 13.22% <0.00%> (-0.01%) ⬇️
examples-hf_ptq 21.48% <50.00%> (-0.04%) ⬇️
examples-llm_distill 13.29% <0.00%> (-0.01%) ⬇️
examples-llm_eval 17.06% <50.00%> (+<0.01%) ⬆️
examples-llm_qat 17.54% <50.00%> (-0.01%) ⬇️
examples-llm_sparsity 15.87% <0.00%> (-0.01%) ⬇️
examples-megatron_bridge 25.67% <0.00%> (-0.15%) ⬇️
examples-specdec_bench 12.97% <0.00%> (-0.01%) ⬇️
examples-speculative_decoding 17.48% <50.00%> (-0.07%) ⬇️
examples-torch_onnx 21.78% <0.00%> (-0.01%) ⬇️
examples-torch_trt 15.03% <0.00%> (-0.01%) ⬇️
gpu 58.50% <66.66%> (-0.71%) ⬇️
regression 14.85% <0.00%> (+0.06%) ⬆️
unit 55.62% <100.00%> (+0.01%) ⬆️

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Signed-off-by: realAsma <akuriparambi@nvidia.com>
Signed-off-by: realAsma <akuriparambi@nvidia.com>
@realAsma
realAsma force-pushed the asma/fix_diffusers_export_dtype branch from 654eeb2 to cc2c218 Compare August 21, 2026 20:41
@realAsma
realAsma marked this pull request as ready for review August 21, 2026 20:41
@realAsma
realAsma requested review from a team as code owners August 21, 2026 20:41
@realAsma
realAsma requested a review from sugunav14 August 21, 2026 20:41

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👉 Steps to fix this

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents
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Inline comments:
In `@tests/unit/torch/export/test_unified_export_hf.py`:
- Around line 36-43: Add focused offline pytest coverage for
_resolve_export_dtype covering configured and unconfigured model dtypes,
including a FrozenDict configuration fixture and the mismatch-warning path.
Reuse local fixtures, assert the resolved dtype for matching and absent
configuration values, and verify that an explicitly requested dtype differing
from the configured dtype emits the expected warning.
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Comment thread tests/unit/torch/export/test_unified_export_hf.py Outdated
Signed-off-by: realAsma <akuriparambi@nvidia.com>

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Inline comments:
In `@tests/unit/torch/export/test_unified_export_hf.py`:
- Around line 40-44: Add a parameterized test case using an empty FrozenDict()
without torch_dtype, with no requested dtype, expecting torch.float32 and the
existing default status value.
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Signed-off-by: realAsma <akuriparambi@nvidia.com>

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Actionable comments posted: 1

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Inline comments:
In `@tests/unit/torch/export/test_unified_export_hf.py`:
- Around line 63-64: Add a brief comment immediately before the
pytest.importorskip call in
test_resolve_export_dtype_with_empty_diffusers_config explaining that Diffusers
is imported locally to keep it optional during torch-only unit-test collection.
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Signed-off-by: realAsma <akuriparambi@nvidia.com>

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LGTM

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Bot review (claude-opus-5) — DM the bot to share feedback.

Small, low-risk change and the added unit tests are welcome, but two things should be resolved before merge:

  1. The new fallback duplicates an existing helper. modelopt/torch/export/diffusers_utils.py::infer_dtype_from_model already implements exactly "first parameter's dtype, else torch.float16" — the same semantics, same magic default. The diffusers export path (_export_diffusers_checkpoint) already calls it. Please reuse it (it's pure torch, so it can be moved to a neutral module if the diffusers-optional import is a problem) instead of inlining a second copy in unified_export_hf.py.

  2. The stated motivation doesn't obviously match the code path. _resolve_export_dtype is only called from _export_transformers_checkpoint and _export_transformers_checkpoint_streaming. export_hf_checkpoint routes diffusers objects to _export_diffusers_checkpoint before either is reached, and that function never touches model.config.torch_dtype. A diffusers pipeline/component can only reach this line if is_diffusers_object() returned False (e.g. the from diffusers import ... guard in unified_export_hf.py / diffusers_utils.py failed under the minimum-transformers combination) — in which case the real bug is the detection/import, and this patch just moves the crash further down the transformers-only path (TiedWeightMap, requantize_resmooth_fused_llm_layers, ... on a non-transformers module). Please paste the actual traceback in the PR body and confirm which path is failing.

Minor: the torch.float16 no-parameter branch is untested, and there is a small behavior change for dtype=None + torch_dtype=None (previously returned None, i.e. weight.to(None) was a no-op preserving per-tensor dtypes; now everything is cast to the first parameter's dtype). That is probably what you want, but it's worth stating explicitly.

No licensing changes and no prompt-injection content in the PR text.

if configured_dtype is not None:
return configured_dtype
first_parameter = next(model.parameters(), None)
return first_parameter.dtype if first_parameter is not None else torch.float16

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Bot comment.

This is a verbatim re-implementation of infer_dtype_from_model() in modelopt/torch/export/diffusers_utils.py (first parameter dtype, torch.float16 fallback), which the diffusers export path already uses via _export_diffusers_checkpoint. Please call that helper here rather than adding a second copy — otherwise the two defaults can drift apart. If importing it from diffusers_utils is awkward because that module is diffusers-optional, note that the function itself has no diffusers dependency and could be moved to a neutral module (e.g. model_utils.py).


def _resolve_export_dtype(model: nn.Module, dtype: torch.dtype | None) -> torch.dtype:
"""Return the export dtype, defaulting to the model's own and warning on a mismatch."""
configured_dtype = getattr(model.config, "torch_dtype", None)

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Bot comment.

How does a Diffusers export reach this function? export_hf_checkpoint() dispatches is_diffusers_object(model) to _export_diffusers_checkpoint(), which resolves dtype through infer_dtype_from_model() and never reads model.config.torch_dtype; _resolve_export_dtype is only reachable from the transformers (resident + streaming) paths. If a diffusers pipeline/component is landing here, that means is_diffusers_object() returned False (most likely HAS_DIFFUSERS/_HAS_DIFFUSERS being False because the from diffusers import ... guard failed under the minimum-transformers combo), and the root cause is the detection, not the missing torch_dtype — this patch would only push the failure to the next transformers-only step (TiedWeightMap(model), requantize_resmooth_fused_llm_layers(model), model.named_modules() on a pipeline). Please include the traceback in the PR body so the actual failing dispatch is on record.

Also, getattr(model.config, ...) still raises if the object has no .config at all; getattr(getattr(model, "config", None), "torch_dtype", None) would be the fully defensive form if that's the intent.

)


def test_resolve_export_dtype_with_empty_diffusers_config():

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Bot comment.

This case is behaviorally identical to the already-parameterized (None, None, torch.float32, 0) row: an empty FrozenDict has no torch_dtype attribute, so both take the next(model.parameters()) branch. What isn't covered is (a) a config that does carry torch_dtype as a string ("bfloat16") — which is how diffusers FrozenDicts deserialize it from JSON, and which would be returned as-is here and then fed to weight.to(...), and (b) the parameterless torch.float16 fallback. Both are worth a row.

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