[SWDEV-538312] Fix num_stages=8 configs for flex attention (#2277)

num_stages==8 configs are always skipped causing breakages

Example error:
```
torch._inductor.exc.LoweringException: NoValidChoicesError: No choices to select, please consider adding ATEN into max_autotune_gemm_backends config (defined in torch/_inductor/config.py) to allow at least one choice. 
  target: flex_attention_backward
```

Author: jataylo

torch/_inductor/kernel/flex_attention.py

@@ -2273,9 +2273,6 @@ def flex_attention_backward(*args, **kwargs):
             or SPARSE_Q_BLOCK_SIZE % BLOCK2 != 0
         ):
             continue
-        if num_warps == 8:
-            # Working around https://github.com/pytorch/pytorch/issues/141603
-            continue
 
         # Performance tuning
         cur_kernel_options = original_kernel_options.copy()