[P/D][KVPool]Mooncake Layerwise Connector supports kv_pool (#7032)

### What this PR does / why we need it?
This PR creates and registers `ascend_multi_connector`, which allows the
`mooncake_layerwise_connector` to use the kv_pooling feature.
We unregister the original vllm's `MultiConnector` and replace it with
`AscendMultiConnector` when registering the connectors.

### Does this PR introduce _any_ user-facing change?
No. User can use `MultiConnector` to initialize `AscendMultiConnector`.

### How was this patch tested?
By CI.

- vLLM version: v0.16.0
- vLLM main:
4034c3d32e

---------

Signed-off-by: nwpu-zxr <zhouxuerong2@huawei.com>

vllm_ascend/distributed/kv_transfer/__init__.py

@@ -19,6 +19,13 @@ from vllm.distributed.kv_transfer.kv_connector.factory import KVConnectorFactory
 
 
 def register_connector():
+    # override multi_connector as ascend_multi_connector
+    if "MultiConnector" in KVConnectorFactory._registry:
+        KVConnectorFactory._registry.pop("MultiConnector")
+    KVConnectorFactory.register_connector(
+        "MultiConnector", "vllm_ascend.distributed.kv_transfer.ascend_multi_connector", "AscendMultiConnector"
+    )
+
     KVConnectorFactory.register_connector(
         "MooncakeConnectorV1", "vllm_ascend.distributed.kv_transfer.kv_p2p.mooncake_connector", "MooncakeConnector"
     )