Fix float16 trunc division rounding

[αI (Alphaxiaoteng)]

Summary

Fixes #990.

ONNX Runtime can evaluate the float16 division in aten::div(..., rounding_mode="trunc") with a higher-precision intermediate than PyTorch. Applying truncation to that value can produce an off-by-one result at float16 boundaries.

Materialize the float16 quotient before truncation, matching PyTorch's float16 rounding semantics. The existing float16 trunc_rounding regression case is enabled again.

Tests

• python -m pytest tests/function_libs/torch_lib/ops_test.py -k test_output_match_opinfo__div_mode -vv
• ruff check onnxscript/function_libs/torch_lib/ops/core.py tests/function_libs/torch_lib/ops_test_data.py
• ruff format --check onnxscript/function_libs/torch_lib/ops/core.py tests/function_libs/torch_lib/ops_test_data.py
• git diff --check

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