[bugfix] Fix mooncake kvpool accuracy issue (#4976)

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

The current KVPool has a accuracy issue
https://github.com/vllm-project/vllm-ascend/issues/4412. This PR aims to
fix the precision problem without impacting prefill performance.

Note：Due to a bug in ADXL, calling `current_event.synchronize()` may
occasionally hang. This issue will be fixed in Cann version 8.5.rc1. You
can manually build the master branch of the project at
https://gitcode.com/cann/hixl to resolve this issue before the 8.5.RC1
release.


- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

---------

Signed-off-by: LCAIZJ <leichao139636@163.com>

vllm_ascend/distributed/kvpool/config_data.py

@@ -1,6 +1,7 @@
 from dataclasses import dataclass
 from typing import Iterable, List, Optional, Tuple, Union
 
+import torch
 from vllm.distributed.kv_transfer.kv_connector.v1.base import \
     KVConnectorMetadata
 from vllm.logger import logger
@@ -284,6 +285,8 @@ class ReqMeta:
 
     is_last_chunk: Optional[bool] = None
 
+    current_event: Optional[torch.npu.Event] = None
+
     @staticmethod
     def from_request_tracker(
         tracker: RequestTracker,
@@ -375,3 +378,4 @@ class LasyerMultiBlockReqMeta:
     block_ids: list[int]
     layer_id: int
     is_last_chunk: Optional[bool] = True
+    current_event: Optional[torch.npu.Event] = None