feat: Add export override options to winml eval command

[xieofxie]

Summary

Brings winml eval to parity with winml build and winml perf by adding four export override options:

• --shape-config — JSON shape overrides for the auto-generated HuggingFace export config
• --input-specs — JSON input tensor specs merged into the HuggingFace export config (symbolic dims infer dynamic axes)
• --export-config — JSON ONNX export config overrides (opset version, constant folding, etc.)
• --dynamic-axes — JSON dynamic axes mapping for HuggingFace ONNX export

These options shape the ONNX export that eval generates when -m is a HuggingFace model ID. When -m is a pre-built .onnx file there is no export step, so the flags are ignored and the command prints a warning.

Changes

• eval/config.py — added shape_config and export_overrides fields to WinMLEvaluationConfig, JSON-safe serialization of InputTensorSpec values, and to_dict/from_dict support.
• eval/evaluate.py — _load_model threads {"export": ...} overrides plus shape_config into WinMLAutoModel.from_pretrained on the HuggingFace build path; the from_onnx path is unchanged.
• commands/eval.py — added the four option decorators/params and an _apply_export_overrides helper that parses the JSON and warns + skips for pre-built ONNX inputs.
• docs/commands/eval.md — documented the four flags and added a pitfall note.
• Tests — added TestEvalExportOverrides (10 cases) and updated the existing from_pretrained assertion for the new shape_config kwarg.

Verification

• uv run pytest tests/unit/commands/test_eval.py tests/unit/eval/test_eval.py → 103 passed
• uv run ruff check → clean
• Doc flag/code alignment check → passing