[v0.18.0][Bugfix] Fixed wrong class attribute assignment (#7586) (#7655)

### What this PR does / why we need it?
Fixed incorrect class attribute assignment and corrected it to instance
attribute assignment. Ensured reorder_batch_threshold only applies to
the current instance to avoid global pollution and multi-instance
conflicts.

Backport of #7586

Signed-off-by: LookAround <lixushi@huawei.com>
Signed-off-by: Mengqing Cao <cmq0113@163.com>
Co-authored-by: LookAround0301 <lixushi@huawei.com>

vllm_ascend/attention/context_parallel/attention_cp.py

@@ -15,8 +15,6 @@
 # This file is a part of the vllm-ascend project.
 #
 
-from typing import ClassVar
-
 import numpy as np
 import torch
 import torch.distributed as dist
@@ -61,11 +59,6 @@ class AscendAttentionCPMetadataBuilder(AscendAttentionMetadataBuilder):
     Extends AscendAttentionMetadataBuilder with PCP/DCP metadata handling.
     """
 
-    # Does this backend/builder reorder the batch?
-    # If not, set this to None. Otherwise set it to the query
-    # length that will be pulled into the front of the batch.
-    reorder_batch_threshold: ClassVar[int] = 1
-
     def __init__(
         self,
         kv_cache_spec: AttentionSpec,