[Bugfix] Fix the attn_metadata is None (#5038)

### What this PR does / why we need it?
Fix the bug " TypeError: 'NoneType' object is not iterable' " in
vllm_ascend/compilation/acl_graph.py
The reason of that is the attn_metadata is none in the dummy_run of MTP.

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

Signed-off-by: chenmenglong <chenmenglong1@huawei.com>

vllm_ascend/spec_decode/interface.py

@@ -32,7 +32,7 @@ class Proposer:
     def dummy_run(self,
                   num_tokens: int,
                   with_prefill: bool = False,
-                  skip_attn: bool = False,
+                  in_graph_capturing: bool = False,
                   num_reqs: int = 0,
                   num_tokens_across_dp: Optional[torch.Tensor] = None,
                   aclgraph_runtime_mode: CUDAGraphMode = CUDAGraphMode.NONE,