[PD][HeteroArch]Fix accuracy issue with CPU_ATTN as Decoder and Flash_ATTN as prefiller (#38935)

Signed-off-by: Chendi Xue <chendi.xue@intel.com>

tests/v1/kv_connector/unit/test_nixl_connector.py

@@ -523,6 +523,7 @@ class FakeNixlConnectorWorker(NixlConnectorWorker):
                     kv_cache_layout="HND",
                     block_size=self.block_size,
                     ssm_sizes=(0, 0),
+                    attn_backend_name=self.backend_name,
                 ),
                 remote_tp_rank=remote_tp_rank,
                 remote_tp_size=remote_tp_size,
@@ -972,6 +973,7 @@ class TestNixlHandshake:
                 kv_cache_layout=mismatched_layout,
                 block_size=worker.block_size,
                 ssm_sizes=(0, 0),
+                attn_backend_name=worker.backend_name,
             )
 
             with pytest.raises(RuntimeError):
@@ -1028,6 +1030,7 @@ class TestNixlHandshake:
                 kv_cache_layout="HND",
                 block_size=worker.block_size,
                 ssm_sizes=(0, 0),
+                attn_backend_name=worker.backend_name,
             )
 
             # We don't check layout for homogeneous TP and MLA for now, as the
@@ -2347,6 +2350,7 @@ def test_compatibility_hash_validation(
         kv_cache_layout="HND",
         block_size=prefill_block_size,
         ssm_sizes=(0, 0),
+        attn_backend_name=decode_worker.backend_name,
     )
     handshake_payload = NixlHandshakePayload(
         compatibility_hash=remote_hash,