re-enable decode tests (#4946)

Summary:
X-link: facebookresearch/FBGEMM#1966

Pull Request resolved: #4946

Previously we disabled the test because it would crash. It seemed to be caused by tensorwise scaled_mm, but we couldn't bisect.

After buck2 clean, it seems working again.

Also asking in https://fb.workplace.com/groups/hm.sup/posts/1902458160316164/, in case it was caused by a driver upgrade.

Reviewed By: q10

Differential Revision: D83501117

fbshipit-source-id: 09c97a197f344490204ae7386dc005ce599026a4

Author: henrylhtsang

fbgemm_gpu/experimental/gen_ai/test/attention/blackwell_fmha_test.py

@@ -431,7 +431,6 @@ def _execute_cutlass_blackwell_attn_varlen(
             assert torch.equal(dk, dk_d)
             assert torch.equal(dv, dv_d)
 
-    @unittest.skipIf(True, "Skip this until we figure out an issue with scaled_mm")
     @skip_cuda_lt_sm100
     @skip_rocm
     @parameterized.expand(