[FIX][Intel GPU]fix ipex flash_attn_varlen_func api missing parameter (#20348)

Signed-off-by: Kunshang Ji <kunshang.ji@intel.com>

vllm/_ipex_ops.py

@@ -270,6 +270,7 @@ class ipex_ops:
         q_descale=None,
         k_descale=None,
         v_descale=None,
+        num_splits=0,
     ):
         if cu_seqlens_k is None:
             # cu_seqlens_k is not used in ipex kernel.