FMHA sink: don't double-scale sink bias

The sink bias from the checkpoint is already in the scaled domain
(added to QK*scale in the reference softmax). The kernel's
running_max is max(QK*scale), so the sink should be compared
directly without multiplying by scale again.

dsv4/kernels/attention/fmha_6warp_tma_multirow_multitile.cuh

@@ -349,7 +349,10 @@ fmha_6warp_tma_multirow_multitile_kernel(FmhaTmaMultiRowMultiTileParams params)
             // Load per-head sink bias (same for all rows in this head)
             float sb = params.sink_bias[head_idx + batch_idx * params.n_h];
             if (my_row_active) {
-                float sink_logit = sb * scale;
+                // sink_bias is already in the scaled domain (added to QK*scale in softmax)
+                // Do NOT multiply by scale again — the kernel's softmax already applies
+                // scale to QK values, and running_max is in the scaled domain.
+                float sink_logit = sb;
                 float old_max = sRunningMax[my_row];
                 float new_max = fmaxf(old_max, sink_logit);
                 float rescale_old = (old_max > -INFINITY) ? expf(old_max - new_max) : 0.0f;