[Refactor] Fix AttentionMaskBuilder singleton and remove redundant pcp_prefill_mask (#4870)

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

This PR fixes the `AttentionMaskBuilder` singleton initialization issue
introduced in PR #4779 and removes the unused `pcp_prefill_mask` field.

### Background

After PR #4779 made `AttentionMaskBuilder` a singleton with `@singleton`
decorator, the class constructor now requires a `device` parameter.
However, two initialization sites were still using the old parameterless
constructor, causing failures.

### Changes

1. **Fix singleton initialization**
- Fixed `AttentionMaskBuilder()` → `AttentionMaskBuilder(self.device)`
in `AscendMLAMetadataBuilder.__init__()`
- Fixed `AttentionMaskBuilder()` → `AttentionMaskBuilder(self.device)`
in `AscendAttentionMetadataBuilder.__init__()`

2. **Remove unused field**
- Removed `pcp_prefill_mask` field from
`AscendPrefillContextParallelMetadata` (never used in codebase)
   - Updated related test assertions

### Related

- Issue #5463
- PR #4779 (Unify all mask generation methods)
- PR #5389 (Make AttentionMaskBuilder singleton)

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

No. This is an internal refactoring.

## How was this patch tested?

- ✅ Local testing: No linter errors
- ✅ Unit tests for attention modules verified
- ⏳ CI pipeline

Signed-off-by: lico67373 <918688502@qq.com>
Co-authored-by: weijinqian0 <1184188277@qq.com>

vllm_ascend/attention/attention_v1.py

@@ -34,6 +34,7 @@ from vllm.v1.attention.backends.utils import (AttentionCGSupport,
 from vllm.v1.core.sched.output import SchedulerOutput
 from vllm.v1.kv_cache_interface import AttentionSpec
 
+from vllm_ascend.attention.attention_mask import AttentionMaskBuilder
 from vllm_ascend.attention.context_parallel.common_cp import (
     AscendMetadataForDecode, AscendMetadataForPrefill)
 from vllm_ascend.attention.utils import (AscendCommonAttentionMetadata,
@@ -219,6 +220,7 @@ class AscendAttentionMetadataBuilder(AttentionMetadataBuilder[AscendMetadata]):
 
         scheduler_config = vllm_config.scheduler_config
         self.chunked_prefill_enabled = scheduler_config.enable_chunked_prefill
+        self.attn_mask_builder = AttentionMaskBuilder(self.device)
 
     @classmethod
     def get_cudagraph_support(
@@ -253,10 +255,19 @@ class AscendAttentionMetadataBuilder(AttentionMetadataBuilder[AscendMetadata]):
         seq_lens = common_attn_metadata.seq_lens_cpu[:num_reqs]
 
         slot_mapping = common_attn_metadata.slot_mapping[:num_actual_tokens]
-        attn_mask = common_attn_metadata.attn_mask
-        swa_mask = common_attn_metadata.swa_mask
         attn_state = common_attn_metadata.attn_state
 
+        # Get attn_mask and swa_mask from singleton AttentionMaskBuilder
+        attn_mask = self.attn_mask_builder.get_attention_mask(
+            self.model_config)
+
+        swa_mask = None
+        is_swa = hasattr(self.model_config.hf_text_config, 'sliding_window')
+        if self.model_config is not None and is_swa:
+            swa_mask = self.attn_mask_builder.get_swa_mask(
+                self.model_config.dtype,
+                self.model_config.hf_text_config.sliding_window)
+
         # TODO: Yet another unnecessary H2D while we already have a query_start_loc on device
         query_start_loc = query_start_loc_cpu.pin_memory().to(
             self.device, non_blocking=True)