[Bug] test_nvfp4_gather_grouped_gemm_act_fusion_blackwell flaky on B300: ~0.05% mismatch vs rtol 1e-4, varying parametrizations

[chang-l]

Summary

unittest/_torch/thop/parallel/test_cute_dsl_moe.py::test_nvfp4_gather_grouped_gemm_act_fusion_blackwell fails intermittently in pre-merge stage B300-PyTorch-2 with:

Exception: Mismatch percentage is ~0.05 for rtol 0.000100

Observed twice on unrelated PR #15730 with different parametrizations:

• [8192-8-1-256-relu2] — L0_MergeRequest_PR build 47303 (mismatch 0.050421)
• [128-1-32-128-relu2] — L0_MergeRequest_PR build 47362 (mismatch 0.054688)

Both runs were on current main bases; the test passes in other runs, so this is a marginal-tolerance flake on B300 rather than a deterministic regression.

Impact

Pre-merge blocker (fail-fast) for unrelated PRs when B300-PyTorch-2 is selected and the tolerance coin-flip loses.

Ask

Loosen rtol / deflake the kernel comparison for B300, or waive until fixed.