Loosen fp16 atol in ONNX export tests for onnxruntime 1.29 CPU numerics

[pggPL]

Description

L1_pytorch_onnx_unittest fails in nightly CI since 2026-08-18 on the fp16 (precision1) variants of test_export_linear_recipe, test_export_layernorm_linear_recipe and test_export_layernorm_mlp. The trigger is the onnxruntime 1.28.0 → 1.29.0 bump in the unpinned CI image: ORT executes the exported graph on the CPU EP, and 1.29 changed fp16 CPU numerics, so the ORT-vs-TE mismatch now reaches up to ~9e-3 (2–9 fp16 ULPs) against the hardcoded atol=1e-3. Verified by A/B on the same TE/torch: ORT 1.28 passes, ORT 1.29 fails; no TE commit is involved.

Since the comparison is half-precision GPU (TE) vs half-precision CPU (ORT), differences of a few ULPs from accumulation order are expected and do not indicate an incorrect ONNX representation. This PR makes the tolerance precision-aware: atol=2e-2 for fp16 (the value already used for FP8 CurrentScaling and FP8 MLP in the same file), keeping 1e-3 for fp32.

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

• Add get_atol(precision) helper in tests/pytorch/test_onnx_export.py returning 2e-2 for fp16 and the default 1e-3 otherwise.
• Use it in the non-FP8 validation paths of _test_export_linear, _test_export_layernorm_linear, _test_export_layernorm_mlp and in the DelayedScaling branch of _test_export_layernorm_linear.

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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