[Bugfix]The service fails to be started when the memcache pool is enabled (#6229)

### What this PR does / why we need it?
The service fails to be started when the memcache pool is enabled
without configuring the mooncake path.

### Does this PR introduce _any_ user-facing change?
NA

### How was this patch tested?
```
#memcache
echo 200000 > /proc/sys/vm/nr_hugepages
source /usr/local/memfabric_hybrid/set_env.sh
source /usr/local/memcache_hybrid/set_env.sh
source /usr/local/Ascend/ascend-toolkit/set_env.sh
source /usr/local/Ascend/nnal/atb/set_env.sh
export MMC_LOCAL_CONFIG_PATH=/usr/local/memcache_hybrid/latest/config/mmc-local.conf

vllm serve /mnt/weight/DeepSeek-V3.2-Exp-W8A8 \
  --host $local_ip \
  --port 8002 \
  --served-model-name model \
  --data-parallel-size 2 \
  --tensor-parallel-size 8 \
  --enable-expert-parallel \
  --no-enable-prefix-caching \
  --no-enable-chunked-prefill \
  --max-num-seqs 4 \
  --max-model-len 8192 \
  --max-num-batched-tokens 8192 \
  --gpu-memory-utilization 0.9 \
  --trust-remote-code \
  --enforce-eager \
  --quantization ascend \
  --additional_config '{"ascend_scheduler_config":{"enabled":false}}' \
  --kv-transfer-config \
    '{
            "kv_connector": "AscendStoreConnector",
            "kv_role": "kv_both",
            "kv_connector_extra_config": {
	            "backend": "memcache",
                "lookup_rpc_port":"0"
            }
    }'
```

- vLLM version: v0.14.0
- vLLM main:
d68209402d

---------

Signed-off-by: lty <linhebiwen@gmail.com>

vllm_ascend/distributed/kv_transfer/kv_pool/ascend_store/pool_worker.py

@@ -1,6 +1,7 @@
+import importlib
 import math
 import threading
-from collections.abc import Callable, Generator
+from collections.abc import Generator
 
 import torch
 from vllm.config import VllmConfig
@@ -14,9 +15,6 @@ from vllm.distributed import (
 from vllm.logger import logger
 from vllm.v1.core.kv_cache_utils import BlockHash
 
-from vllm_ascend.distributed.kv_transfer.kv_pool.ascend_store.backend.backend import Backend
-from vllm_ascend.distributed.kv_transfer.kv_pool.ascend_store.backend.memcache_backend import MemcacheBackend
-from vllm_ascend.distributed.kv_transfer.kv_pool.ascend_store.backend.mooncake_backend import MooncakeBackend
 from vllm_ascend.distributed.kv_transfer.kv_pool.ascend_store.config_data import (
     AscendConnectorMetadata,
     ChunkedTokenDatabase,
@@ -32,9 +30,15 @@ from vllm_ascend.distributed.kv_transfer.kv_pool.ascend_store.kv_transfer import
     KVTransferThread,
 )
 
-backend_map: dict[str, Callable[..., Backend]] = {
-    "mooncake": MooncakeBackend,
-    "memcache": MemcacheBackend,
+backend_map = {
+    "mooncake": {
+        "name": "MooncakeBackend",
+        "path": "vllm_ascend.distributed.kv_transfer.kv_pool.ascend_store.backend.mooncake_backend",
+    },
+    "memcache": {
+        "name": "MemcacheBackend",
+        "path": "vllm_ascend.distributed.kv_transfer.kv_pool.ascend_store.backend.memcache_backend",
+    },
 }
 
 
@@ -125,7 +129,13 @@ class KVPoolWorker:
 
         self.token_database = ChunkedTokenDatabase(self.metadata, self.block_size, self.use_mla, partitions)
 
-        real_backend = backend_map.get(self.backend.lower())
+        backend = backend_map.get(self.backend.lower())
+        assert backend is not None
+        backend_path = backend.get("path")
+        backend_name = backend.get("name")
+        assert backend_path is not None and backend_name is not None
+        backend_module = importlib.import_module(backend_path)
+        real_backend = getattr(backend_module, backend_name)
 
         # be removed later
         if self.backend == "mooncake":