[Refactor] Import global var form vllm instead of overwirte it (#5469)

### What this PR does / why we need it?
Import global var form vllm instead of overwirte it, so that we could
use the correct global variant value

- vLLM version: v0.13.0
- vLLM main:
5326c89803
---------
Signed-off-by: MengqingCao <cmq0113@163.com>

vllm_ascend/ops/triton/mamba/causal_conv1d.py

@@ -13,8 +13,7 @@ import torch
 import torch.nn.functional as F
 import triton
 import triton.language as tl
-
-PAD_SLOT_ID = -1
+from vllm.attention.backends.utils import PAD_SLOT_ID
 
 
 def causal_conv1d_ref(