fix: Unreachable backend check after earlier backend skip

[andrewwhitecdw]

This PR removes an unreachable backend skip from the JAX fused-attention test config checks and adds a regression test.

Root cause

In FusedAttnRunner._check_configs (tests/jax/test_fused_attn.py), the backend is selected via FusedAttnHelper and any backend other than NVTE_Fused_Attn_Backend.NVTE_F16_arbitrary_seqlen is skipped immediately after selection ("Unsupported inputs combination or device compute capability."). Inside the subsequent non-1HSS post-scale-bias block, the elif re-testing self.backend != NVTE_Fused_Attn_Backend.NVTE_F16_arbitrary_seqlen could therefore never be true, making its skip message dead code.

Changes

• tests/jax/test_fused_attn.py: removed the unreachable elif arm; the sibling padding-mask skip is retained unchanged.
• tests/jax/test_fused_attn.py: added test_post_scale_non_1hss_bias_shapes_do_not_hit_unreachable_backend_skip, which builds a non-1HSS post-scale-bias config (BiasShape._B1SS, NO_MASK, BSHD layout) whose backend is NVTE_F16_arbitrary_seqlen, calls _check_configs() without hitting a skip, and asserts the selected backend.

Behavior

Behavior on real runs is unchanged: configurations that previously passed still pass, no configuration ever reached the removed arm, and the padding-mask restriction for non-1HSS post-scale bias shapes is preserved.

Testing

Not run locally — the JAX test stack (jax, transformer_engine_jax) is not installed on this machine, so the JAX suite could not be executed here; CI runs it. The added test follows the file's existing pattern of targeting real GPU/cuDNN fused-attention kernels (such tests skip themselves when the hardware/software backend is unavailable).

Branch

Recreated as a single signed-off commit on current main (DCO) in response to the review request.

[greptile-apps]

Greptile Summary

This PR removes an unreachable fused-attention backend check from the JAX test configuration logic.

• Keeps the earlier backend gate as the sole backend eligibility check.
• Preserves the non-1HSS padding-mask restriction.
• The regression test described by the PR has been removed from the current revision, leaving the changed path without focused collected coverage.

Confidence Score: 5/5

The runtime cleanup appears safe to merge, but restoring focused regression coverage would prevent the unreachable check from being reintroduced unnoticed.

The backend check was redundant behind the existing arbitrary-sequence-length backend gate, and no blocking behavioral defect remains; the sole new concern is the non-blocking loss of targeted regression coverage.

Files Needing Attention: tests/jax/test_fused_attn.py

Important Files Changed

Filename	Overview
tests/jax/test_fused_attn.py	Removes a redundant backend skip, but also removes the only focused collected regression test for the affected configuration path.

Comments Outside Diff (1)

1. tests/jax/test_fused_attn.py, line 1414-1447 (link)

   Regression coverage removed

   This deletes the only collected regression test that exercises a non-1HSS post-scale bias with NO_MASK through _check_configs. The remaining non-1HSS cases are in non-collected helper methods, so CI would not detect this unreachable backend check being reintroduced. Please retain a focused collected test for this path.

   Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

_{Reviews (5): Last reviewed commit: "Remove unnecessary test" | Re-trigger Greptile}

[cyanguwa]

This is a no-brainer. Thanks.

[andrewwhitecdw]

script closed on accident

[andrewwhitecdw]

script closed on accident

[ptrendx]

@andrewwhitecdw Please sign your commits.