[model][utils] add extract_layer_index utility function (#10599)

Signed-off-by: youkaichao <youkaichao@gmail.com>

vllm/model_executor/models/utils.py

@@ -629,3 +629,24 @@ def maybe_prefix(prefix: str, name: str) -> str:
         The string "prefix.name" if prefix was non-empty, otherwise just "name".
     """
     return name if not prefix else f"{prefix}.{name}"
+
+
+def extract_layer_index(layer_name: str) -> int:
+    """
+    Extract the layer index from the module name.
+    Examples:
+    - "encoder.layers.0" -> 0
+    - "encoder.layers.1.self_attn" -> 1
+    - "2.self_attn" -> 2
+    - "model.encoder.layers.0.sub.1" -> ValueError
+    """
+    subnames = layer_name.split(".")
+    int_vals: List[int] = []
+    for subname in subnames:
+        try:
+            int_vals.append(int(subname))
+        except ValueError:
+            continue
+    assert len(int_vals) == 1, (f"layer name {layer_name} should"
+                                " only contain one integer")
+    return int_vals[0]