Fix composite model export (Qwen3 and seq2seq models)

[xieofxie]

Summary

Refactors winml export to support composite (seq2seq) models such as Qwen3, T5, and BART. Previously, running winml export on a composite model would fail because the command treated every model as a single-component export. Now, composite models are automatically detected and each sub-component (e.g. encoder / decoder / decoder-with-past) is exported separately into its own ONNX file.

Changes

src/winml/modelkit/loader/resolution.py

• Added resolve_composite_components() — a shared entry point that resolves a composite model's _SUB_MODEL_CONFIG (sub-name → task mapping). Works with both explicit --task and auto-detected task paths.

src/winml/modelkit/commands/export.py

• Extracted per-component export logic into _run_component_export() inner function (I/O tensor resolution, config assembly, model load, and ONNX export).
• Added composite detection: when a model is composite, the command fans out into one export per sub-component, writing each to a stem-suffixed flat path <output_stem>_<sub_name>.onnx (matching the sibling winml config fan-out and the runtime's *_model.onnx discovery).
• Non-composite models continue through the single-export path unchanged.
• Intentional loud guards during composite resolution (empty registry RuntimeError / model-task incompatibility ValueError) are re-raised (the latter as a UsageError, mirroring winml config) instead of being masked as "not composite".
• Each composite sub-component is loaded and exported independently because a composite's sub-tasks map to distinct model classes/heads; the per-component reload is intentional and documented inline.

src/winml/modelkit/commands/config.py

• Switched to the new shared resolve_composite_components() helper, removing duplicated resolution logic.

src/winml/modelkit/export/htp/exporter.py

• Added clear_sidecar() method to HTPExporter for cleaning up external data sidecar files after export.

src/winml/modelkit/onnx/__init__.py

• Exported the new clear_sidecar symbol.

Tests

• Added unit tests for composite export fan-out (flat stem-suffixed layout), the --input-specs rejection, that resolution errors surface instead of being swallowed, and resolve_composite_components.

[DingmaomaoBJTU]

Reviewed the composite-export fan-out and the external-data cleanup. The refactor is clean overall — extracting _run_component_export and the shared resolve_composite_components reads well, and _cleanup_stale_external_data correctly keys off the final on-disk references, so it's safe whether the consolidated re-save ends up inline or external.

A few things worth considering (see inline):

1. For composites, --output is silently treated as a directory, which is inconsistent with the winml config fan-out convention and with this PR's own description.
2. The broad except Exception around composite detection swallows the intentional loud RuntimeError from _composite_registry().
3. The full model is reloaded once per sub-component.

Nits (no inline):

• The PR description says a clear_sidecar() method and a clear_sidecar export were added, but the code actually adds HTPExporter._cleanup_stale_external_data() and exports get_external_data_files. Please sync the description.
• The composite --input-specs UsageError is raised after the Starting HTP export... banner prints; hoisting the check above the banner would read cleaner.
• The new composite tests mock load_hf_model / resolve_export_config / export_onnx, so they verify the fan-out plumbing (paths, call counts) but not that the sub-models actually differ — worth a follow-up integration check.

(Note: I couldn't run the suite locally — this machine is win_arm64 and torch ships no matching wheel, so this is a static review.)

[DingmaomaoBJTU]

I left a few inline comments around composite export error handling, output preflight, and test coverage.

[DingmaomaoBJTU]

I left one follow-up inline comment on the latest update.

[DingmaomaoBJTU]

Approved with the follow-up cleanup comment left as non-blocking.

[xieofxie]

Failure log will be like

Success! Model exported to: temp\export_qwen3_decoder_prefill

Warning: composite export did not finish; 1 sub-model(s) were written/updated by this run:
  • temp\export_qwen3_decoder_prefill
The export did not complete for every sub-model. Review these files and remove them if you don't want to keep the partial export.

Export failed: No active exception to reraise
[14:34:44 ERROR   winml.modelkit.commands.export] Export failed: No active exception to reraise
Error: Export failed: No active exception to reraise