[Model] PP support for embedding models and update docs (#9090)

Co-authored-by: Roger Wang <136131678+ywang96@users.noreply.github.com>

vllm/model_executor/models/utils.py

@@ -306,10 +306,12 @@ def get_pp_missing_layer_names(model: torch.nn.Module) -> List[str]:
 
 def is_pp_missing_parameter(name: str, model: torch.nn.Module) -> bool:
     """Check if a parameter is missing in a pipeline parallel model."""
-    for missing_layer_name in get_pp_missing_layer_names(model):
-        if name.startswith(missing_layer_name):
-            return True
-    return False
+    if isinstance(model, PPMissingLayer):
+        return True
+
+    return any(
+        name.startswith(missing_layer_name)
+        for missing_layer_name in get_pp_missing_layer_names(model))
 
 
 def make_empty_intermediate_tensors_factory(keys: List[str], hidden_size: int):