ConfigError: Field 'app' Not Yet Prepared with ForwardRef Error When Initializing VespaRetriever
Labels: name: area: vector store, name: auto:bug

### System Info

Hello team,
I'm currently facing an issue while trying to use VespaRetriever within the context of langchain: '0.0.281' . 
Vespa has been successfully deployed on my local environment, and queries are functioning correctly

```
from langchain.retrievers.vespa_retriever import VespaRetriever

vespa_query_body = {
    'yql': 'select * from sources * where userQuery()',
    'query': 'what keeps planes in the air',
    'ranking': 'native_rank',
    'type': 'all',
    'hits': 10
}
vespa_content_field = "body"
retriever = VespaRetriever(app=app, body=query, content_field=vespa_content_field)
```

````
---------------------------------------------------------------------------
ConfigError                               Traceback (most recent call last)
Cell In[34], line 11
      3 vespa_query_body = {
      4     'yql': 'select * from sources * where userQuery()',
      5     'query': 'what keeps planes in the air',
   (...)
      8     'hits': 10
      9 }
     10 vespa_content_field = "body"
---> 11 retriever = VespaRetriever(app=app, body=query, content_field=vespa_content_field)

File ~/miniconda3/lib/python3.10/site-packages/langchain/load/serializable.py:75, in Serializable.__init__(self, **kwargs)
     74 def __init__(self, **kwargs: Any) -> None:
---> 75     super().__init__(**kwargs)
     76     self._lc_kwargs = kwargs

File ~/miniconda3/lib/python3.10/site-packages/pydantic/main.py:339, in pydantic.main.BaseModel.__init__()

File ~/miniconda3/lib/python3.10/site-packages/pydantic/main.py:1076, in pydantic.main.validate_model()

File ~/miniconda3/lib/python3.10/site-packages/pydantic/fields.py:860, in pydantic.fields.ModelField.validate()

ConfigError: field "app" not yet prepared so type is still a ForwardRef, you might need to call VespaRetriever.update_forward_refs().
````

```
from vespa.application import Vespa

VespaRetriever.update_forward_refs()
````

````
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
Cell In[37], line 3
      1 from vespa.application import Vespa
----> 3 VespaRetriever.update_forward_refs()

File ~/miniconda3/lib/python3.10/site-packages/pydantic/main.py:815, in pydantic.main.BaseModel.update_forward_refs()

File ~/miniconda3/lib/python3.10/site-packages/pydantic/typing.py:554, in pydantic.typing.update_model_forward_refs()

File ~/miniconda3/lib/python3.10/site-packages/pydantic/typing.py:520, in pydantic.typing.update_field_forward_refs()

File ~/miniconda3/lib/python3.10/site-packages/pydantic/typing.py:66, in pydantic.typing.evaluate_forwardref()

File ~/miniconda3/lib/python3.10/typing.py:694, in ForwardRef._evaluate(self, globalns, localns, recursive_guard)
    689 if self.__forward_module__ is not None:
    690     globalns = getattr(
    691         sys.modules.get(self.__forward_module__, None), '__dict__', globalns
    692     )
    693 type_ = _type_check(
--> 694     eval(self.__forward_code__, globalns, localns),
    695     "Forward references must evaluate to types.",
    696     is_argument=self.__forward_is_argument__,
    697     allow_special_forms=self.__forward_is_class__,
    698 )
    699 self.__forward_value__ = _eval_type(
    700     type_, globalns, localns, recursive_guard | {self.__forward_arg__}
    701 )
    702 self.__forward_evaluated__ = True

File <string>:1

NameError: name 'Vespa' is not defined
```

### Who can help?

_No response_

### Information

- [X] The official example notebooks/scripts
- [ ] My own modified scripts

### Related Components

- [ ] LLMs/Chat Models
- [ ] Embedding Models
- [ ] Prompts / Prompt Templates / Prompt Selectors
- [ ] Output Parsers
- [ ] Document Loaders
- [X] Vector Stores / Retrievers
- [ ] Memory
- [ ] Agents / Agent Executors
- [ ] Tools / Toolkits
- [ ] Chains
- [ ] Callbacks/Tracing
- [ ] Async

### Reproduction

The same example here https://python.langchain.com/docs/integrations/retrievers/vespa where I have a local url that works correctly

### Expected behavior

Retrieve indexed document on vespa.ia