[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/worker/worker.py

@@ -504,6 +504,7 @@ class NPUWorker(WorkerBase):
 
     def initialize_from_config(self, kv_cache_config: KVCacheConfig) -> None:
         """Allocate NPU KV cache with the specified kv_cache_config."""
+        ensure_kv_transfer_initialized(self.vllm_config, kv_cache_config)
         if self.vllm_config.model_config.enable_sleep_mode:
             allocator = CaMemAllocator.get_instance()
             context = allocator.use_memory_pool(tag="kv_cache")
@@ -575,7 +576,6 @@ class NPUWorker(WorkerBase):
             self.parallel_config.decode_context_parallel_size,
         )
         init_ascend_model_parallel(self.parallel_config)
-        ensure_kv_transfer_initialized(self.vllm_config)
         ensure_ec_transfer_initialized(self.vllm_config)
 
     def _create_profiler(self, trace_name: str):