[Bugfix]KV pool rank 0 consumes more HBM (#6113)

### What this PR does / why we need it?

before add_set_deivce
<img width="2354" height="674" alt="image"
src="https://github.com/user-attachments/assets/8b81ab5f-b9ba-4fd2-8546-8f36ac15d32b"
/>
after
<img width="1044" height="156" alt="image"
src="https://github.com/user-attachments/assets/996d845a-8abd-4aae-b894-4a9832b1f742"
/>

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

### How was this patch tested?

- vLLM version: v0.13.0
- vLLM main:
d68209402d

---------

Signed-off-by: baxingpiaochong <771405853@qq.com>

vllm_ascend/distributed/kv_transfer/kv_pool/ascend_store/backend/mooncake_backend.py

@@ -2,6 +2,8 @@
 import json
 import os
 import re
+import torch
+
 from dataclasses import dataclass
 from typing import Union
 
@@ -32,6 +34,7 @@ class MooncakeBackend(Backend):
                 "to run vLLM with MooncakeConnector.") from e
         self.config = MooncakeStoreConfig.load_from_env()
         self.store = MooncakeDistributedStore()
+        self.rank = parallel_config.rank
         if self.config.protocol == "ascend":
             local_hostname = get_ip()
             transfer_engine = global_te.get_transfer_engine(local_hostname,
@@ -50,6 +53,10 @@ class MooncakeBackend(Backend):
             logger.error(msg)
             raise RuntimeError(msg)
 
+    def set_device(self):
+        device = torch.device(f"npu:{self.rank}")
+        torch.npu.set_device(device)
+
     def register_buffer(self, ptrs: list[int], lengths: list[int]):
         global_te.register_buffer(ptrs, lengths)