[1/N][Eagle3] Aligns auxiliary hidden state usage for eagle3 models (#5162)

### What this PR does / why we need it?
This is to prepare for the migration to vLLM's `EagleProposer`, it does
not have `name` attribution. Also it's a breakdown of #5100 .

Introduces logic to determine whether eagle3 heads require auxiliary
hidden states based on configuration, ensuring consistent handling
across related components. Prevents incorrect assumptions for eagle3
variants that do not use auxiliary outputs, improving compatibility and
correctness.

### Does this PR introduce _any_ user-facing change?
None.

### How was this patch tested?
None.
- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

---------

Signed-off-by: Yizhou Liu <liu_yizhou@outlook.com>

vllm_ascend/spec_decode/eagle_proposer.py

@@ -45,6 +45,9 @@ class EagleProposer(Proposer):
         self.vllm_config = vllm_config
         self.device = device
         self.runner = runner
+        self.speculative_config = vllm_config.speculative_config
+        self.draft_model_config = self.speculative_config.draft_model_config
+        self.method = self.speculative_config.method
 
         self.block_size = vllm_config.cache_config.block_size
         # We need to get the hidden size from the draft model config because
@@ -99,6 +102,29 @@ class EagleProposer(Proposer):
                                        device="cpu",
                                        dtype=torch.int32)
         self.attn_mask_builder = AttentionMaskBuilder(self.device)
+        self.eagle3_use_aux_hidden_state: bool = (
+            self._get_eagle3_use_aux_hidden_state_from_config())
+
+    def _get_eagle3_use_aux_hidden_state_from_config(self) -> bool:
+        """
+        NOTE(2025-12-18): This is an explicit copy from vLLM EagleProposer, only added
+        to align with its logics.
+
+        Some eagle3 heads (e.g., nvidia/gpt-oss-120b-Eagle3-v2) do not use auxiliary
+        hidden states and directly uses the last layer output just like eagle1.
+        They might indicate this by setting "use_aux_hidden_state" to False
+        inside the "eagle_config" dict of their hf_config.
+        """
+        if self.method != "eagle3":
+            return False
+        # Assume that eagle3 heads use aux hidden states by default
+        use_aux_hidden_state = True
+        eagle_config = getattr(self.draft_model_config.hf_config,
+                               "eagle_config", None)
+        if eagle_config is not None:
+            use_aux_hidden_state = eagle_config.get("use_aux_hidden_state",
+                                                    True)
+        return use_aux_hidden_state
 
     def load_model(self, model: nn.Module) -> None:
         target_attn_layer_names = set(