fix: Unreachable backend check after earlier backend skip (#3368)

* fix: remove unreachable fused-attn backend skip and add regression test

Root cause: FusedAttnRunner._check_configs skipped with 'Unsupported
inputs combination or device compute capability.' unless the backend was
NVTE_F16_arbitrary_seqlen. The later elif re-testing
self.backend != NVTE_F16_arbitrary_seqlen could therefore never be
reached, so its skip message ('B1SS, BHSS and 11SS bias shapes are only
supported for the F16_arbitrary_seqlen backend') was dead code.

Fix: drop the dead elif arm. The padding-mask skip in the sibling if arm
is retained. A regression test locks the remaining behavior: a
non-1HSS post-scale-bias config (BiasShape._B1SS) that passes
_check_configs selects NVTE_F16_arbitrary_seqlen, proving the removal is
behaviorally invisible and the earlier guard is the sole gate.

Testing: not run locally - the JAX test stack (jax,
transformer_engine_jax) is not installed on this machine. The suite runs
in NVIDIA TransformerEngine CI. Contribution: tests/jax tests target
real GPU/cuDNN fused-attention kernels and skip otherwise; the new test
will follow that path via CI.

Signed-off-by: andrewwhitecdw <andrewwhitecdw@users.noreply.github.com>

* Remove unnecessary test

Signed-off-by: Przemyslaw Tredak <ptrendx@gmail.com>

---------

Signed-off-by: andrewwhitecdw <andrewwhitecdw@users.noreply.github.com>
Signed-off-by: Przemyslaw Tredak <ptrendx@gmail.com>
Co-authored-by: andrewwhitecdw <andrewwhitecdw@users.noreply.github.com>
Co-authored-by: Przemyslaw Tredak <ptrendx@gmail.com>

Author: andrewwhitecdw

tests/jax/test_fused_attn.py

@@ -612,11 +612,6 @@ def _check_configs(self):
                 pytest.skip(
                     "B1SS, BHSS and 11SS bias shapes are only supported for non-padding mask"
                 )
-            elif self.backend != NVTE_Fused_Attn_Backend.NVTE_F16_arbitrary_seqlen:
-                pytest.skip(
-                    "B1SS, BHSS and 11SS bias shapes are only supported for "
-                    "the F16_arbitrary_seqlen backend."
-                )
 
     def _setup_inputs(self):
         self._check_configs()