[XPU] properly handle q_descale on XPU as quant query input not supported (#39676)

Signed-off-by: Yan Ma <yan.ma@intel.com>
Co-authored-by: Kunshang Ji <kunshang.ji@intel.com>

vllm/v1/attention/backends/flash_attn.py

@@ -1031,7 +1031,9 @@ class FlashAttentionImpl(AttentionImpl):
             window_size=sliding_window_size,
             softcap=self.logits_soft_cap,
             fa_version=self.vllm_flash_attn_version,
-            q_descale=layer._q_scale.expand(descale_shape),
+            q_descale=layer._q_scale.expand(descale_shape)
+            if self.supports_quant_query_input
+            else None,
             k_descale=layer._k_scale.expand(descale_shape),
             v_descale=layer._v_scale.expand(descale_shape),
             num_splits=1 if self.batch_invariant_enabled else 0,