[perf][dsv3.2][async_scheduling] improve dsv3.2 performance by eliminating HD synchronization (#4805)

### What this PR does / why we need it?
This PR eliminates the simplicit HD synchronization in sfa backend, and
_build_dummy_attn_metadata and dummy_run in mtp_proposer, significantly
improving dsv3.2 performance in low-latency scenarios.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Performance improvements are observed with E2E performance serving (P:
DP4TP8EP32 D: DP8TP4EP32) with `num_speculative_tokens=3`.

DSV3.2-W8A8-EXP:
TPOT: 41.67ms -> 23.36ms
ITL: 85.93ms -> 55.96ms

DSV3.2-W8A8 (relaesed in December):
TPOT: 18.11ms
ITL: 56.13ms
 

- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

Signed-off-by: linfeng-yuan <1102311262@qq.com>

vllm_ascend/spec_decode/mtp_proposer.py

@@ -233,7 +233,13 @@ class MtpProposer(Proposer):
             num_tokens_across_dp,
             with_prefill,
         ) = self.runner._sync_metadata_across_dp(num_tokens, with_prefill)
-
+        if self.use_async_scheduling:
+            # there is synchronization between mtp steps when enabling aclgraph,
+            # disable aclgraph when use async scheduling to avoid the
+            # synchronization overhead.
+            # NOTE: we need to set aclgraph_runtime_mode to None in both dummy_run
+            # and _propose.
+            aclgraph_runtime_mode = CUDAGraphMode.NONE
         moe_comm_type = self.runner._select_moe_comm_method(num_tokens)
         # TODO: remove this after moe_comm_type selection logic is finalized
         moe_comm_type = (MoECommType.ALLTOALL if moe_comm_type
@@ -742,9 +748,11 @@ class MtpProposer(Proposer):
         aclgraph_runtime_mode, batch_descriptor = \
             self.runner.aclgraph_dispatcher.dispatch(num_tokens=num_input_tokens, uniform_decode=uniform_decode, has_lora=has_lora)
         if self.use_async_scheduling:
-            # there is synchronize between mtp steps when enable aclgraph,
+            # there is synchronization between mtp steps when enabling aclgraph,
             # disable aclgraph when use async scheduling to avoid the
-            # synchronize overhead.
+            # synchronization overhead.
+            # NOTE: we need to set aclgraph_runtime_mode to None in both dummy_run
+            # and _propose.
             aclgraph_runtime_mode = CUDAGraphMode.NONE
 
         if self.vllm_config.compilation_config.cudagraph_mode.has_full_cudagraphs(