[Bugfix] Obtain kernel block size for computing slot mapping correctly (#7019)

### What this PR does / why we need it?
This PR aims to fix incorrect slot mapping in qwen35 due to mismatched
block size. In qwen35, we should use `kernel_block_size` so that we can
compute it in a correct way, and it is obtained in `load_model` when we
have a chance to grab `draft_attn_layers`.

- vLLM version: v0.16.0
- vLLM main:
15d76f74e2

Signed-off-by: Zetong Li <slippersss@126.com>

vllm_ascend/spec_decode/eagle_proposer.py

@@ -161,13 +161,19 @@ class AscendEagleProposer(EagleProposer):
             )
 
         indexer_layers = get_layers_from_vllm_config(self.vllm_config, DeepseekV32IndexerCache).keys()
-        draft_attn_layer = get_layers_from_vllm_config(self.vllm_config, AttentionLayerBase).keys()
+        draft_attn_layers_dict = get_layers_from_vllm_config(self.vllm_config, AttentionLayerBase)
+        draft_attn_layers = draft_attn_layers_dict.keys()
 
-        draft_attn_layer_names = draft_attn_layer - target_attn_layer_names
+        draft_attn_layer_names = draft_attn_layers - target_attn_layer_names
         draft_indexer_layer_names = indexer_layers - target_indexer_layer_names
         draft_attn_layer_names = draft_attn_layer_names - draft_indexer_layer_names
         assert len(draft_attn_layer_names) == 1
         self.attn_layer_names = list(sorted(draft_attn_layer_names))
+
+        self.kernel_block_size = (
+            draft_attn_layers_dict[self.attn_layer_names[0]].get_attn_backend().get_supported_kernel_block_sizes()[0]
+        )
+
         self.piece_all_attn_layer_name = []
         for _ in range(self.num_speculative_tokens):
             self.piece_all_attn_layer_name.append([name for name in self.attn_layer_names])
@@ -978,7 +984,10 @@ class AscendEagleProposer(EagleProposer):
             slot_mapping = mtp_slot_mapping[slot_indices]
             common_attn_metadata.slot_mapping[: batch_size * self.pcp_size] = slot_mapping
         else:
-            block_size = attn_metadata_builder.kv_cache_spec.block_size
+            # NOTE: In vllm, `block_size = attn_metadata_builder.kv_cache_spec.block_size`.
+            # However, in vllm-ascend, the above value can be multiple of `kernel_block_size`,
+            # which is not correct for computing `slot_mapping` below.
+            block_size = self.kernel_block_size
 
             # Compute the slot mapping.
             if self.uses_mrope: