[CPU] Specify required KV cache layout for CPU attention backend (vllm-project#42740)

Signed-off-by: Tony Lin <tony.lin@intel.com>
Co-authored-by: Li, Jiang <jiang1.li@intel.com>

Author: hlin99

vllm/v1/attention/backends/cpu_attn.py

@@ -25,6 +25,7 @@
     MultipleOf,
 )
 from vllm.v1.attention.backends.utils import (
+    KVCacheLayoutType,
     split_decodes_and_prefills,
 )
 from vllm.v1.kv_cache_interface import AttentionSpec, CrossAttentionSpec
@@ -94,6 +95,10 @@ def get_kv_cache_shape(
     ) -> tuple[int, ...]:
         return 2, num_blocks, num_kv_heads, block_size, head_size
 
+    @classmethod
+    def get_required_kv_cache_layout(cls) -> "KVCacheLayoutType | None":
+        return "HND"
+
     @staticmethod
     def use_cascade_attention(*args, **kwargs) -> bool:
         return False