[Feature] Optimize Qwen3.5/Qwen3Next GDN prefill by prebuilding chunk metadata (#7487)

### What this PR does / why we need it?
This PR optimizes the Qwen3.5 and Qwen3Next GDN prefill path on Ascend
by reducing host/device synchronization overhead.

The current implementation of the `chunk_gated_delta_rule` path for
variable-length sequences prepares chunk metadata during the forward
pass. This approach triggers frequent CPU intervention and host/device
round-trips. When running prefill-heavy workloads with asynchronous
scheduling enabled, these synchronizations result in execution "bubbles"
and prefill stalling (stuttering). **Note that this does not cause
asynchronous scheduling to fail; rather, it prevents the system from
reaching its theoretical throughput due to these unnecessary stalls.**

To resolve this, the patch moves metadata preparation out of the hot
path:
- **Prebuilt Metadata:** All non-speculative varlen chunk metadata for
GDN is now prebuilt on the CPU.
- **Asynchronous Transfer:** Staging buffers are kept in pinned memory
and transferred to the NPU asynchronously.
- **Integration:** The prebuilt bundle is attached to GDN attention
metadata via `patch_gdn_attn.py` and passed into Triton wrappers.
- **Backward Compatibility:** Triton wrappers fall back to the legacy
preparation path if no prebuilt metadata is provided.

- vLLM version: v0.17.0
- vLLM main:
8b6325758c
---------
Signed-off-by: maoxx241 <maomaoyu870@gmail.com>

vllm_ascend/patch/worker/patch_qwen3_5.py

@@ -178,6 +178,11 @@ class AscendQwen3_5GatedDeltaNet(Qwen3_5GatedDeltaNet):
             if attn_metadata.num_prefills > 0:
                 initial_state = ssm_state[non_spec_state_indices_tensor].contiguous()
                 initial_state[~has_initial_state, ...] = 0
+                non_spec_chunked_prefill_meta = getattr(
+                    attn_metadata,
+                    "non_spec_chunked_prefill_meta",
+                    None,
+                )
                 (
                     core_attn_out_non_spec,
                     last_recurrent_state,
@@ -190,6 +195,7 @@ class AscendQwen3_5GatedDeltaNet(Qwen3_5GatedDeltaNet):
                     initial_state=initial_state,
                     output_final_state=True,
                     cu_seqlens=non_spec_query_start_loc,
+                    prebuilt_meta=non_spec_chunked_prefill_meta,
                     head_first=False,
                     use_qk_l2norm_in_kernel=True,
                 )