[Bugfix][CT] Fix KV cache scale handling (#39418)

Signed-off-by: yiliu30 <yi4.liu@intel.com>

vllm/model_executor/layers/quantization/compressed_tensors/compressed_tensors.py

@@ -1123,6 +1123,17 @@ class CompressedTensorsKVCacheMethod(BaseKVCacheMethod):
         layer._v_scale = layer.v_scale
         layer._q_scale = layer.q_scale
 
+        # Set the _float variants that the attention backend uses.
+        def _to_scalar(tensor: torch.Tensor) -> float:
+            # For n_scales > 1 (e.g., ATTN_HEAD strategy), take max
+            if tensor.numel() > 1:
+                return tensor.max().item()
+            return tensor.item()
+
+        layer._k_scale_float = _to_scalar(layer.k_scale)
+        layer._v_scale_float = _to_scalar(layer.v_scale)
+        layer._q_scale_float = _to_scalar(layer.q_scale)
+
         # Discard all placeholders.
         del layer.k_scale
         del layer.v_scale