[Perf] Delete redundant operations in model_runner and forward_context (#3677)

### What this PR does / why we need it?

Remove redundant operations from `model_runner` and `forward_context`.
This optimization can significantly reduce the idle time (bubble) before
decoding when running models with small parameter counts (e.g.,
Qwen/Qwen2.5-0.5B).

Testing on 800I A2, bubble is reduced from 3.8ms to 2.8ms :
Before
<img width="1655" height="696" alt="image"
src="https://github.com/user-attachments/assets/d7608e52-2438-46dd-8fc9-391fd6274495"
/>

After
<img width="1607" height="774" alt="image"
src="https://github.com/user-attachments/assets/56daf081-2dba-4d2e-99d4-e055187d9806"
/>

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?


- vLLM version: v0.11.0rc3
- vLLM main:
https://github.com/vllm-project/vllm/commit/releases/v0.11.1

---------

Signed-off-by: realliujiaxu <realliujiaxu@163.com>

vllm_ascend/utils.py

@@ -58,6 +58,7 @@ _DEFAULT_BUFFER_SIZE = 200
 _MIN_DP_BUFFER_SIZE = 50
 _IS_MOE_MODEL = None
 _ENABLE_SP = None
+_HAS_LAYER_IDX = None
 
 
 def is_310p():
@@ -807,3 +808,14 @@ def version_check():
         if full_date >= "20250919":
             return True
     return False
+
+
+def has_layer_idx(model_instance: torch.nn.Module) -> bool:
+    if model_instance is None:
+        return False
+
+    global _HAS_LAYER_IDX
+    if _HAS_LAYER_IDX is None:
+        _HAS_LAYER_IDX = hasattr(model_instance, "model") and \
+            hasattr(model_instance.model, "start_layer")
+    return _HAS_LAYER_IDX