### 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
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Signed-off-by: LCAIZJ <leichao139636@163.com>
9c02fa9867