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.