Vector Search on GCP Private Network gives AttributeError: 'MatchingEngineIndexEndpoint' object has no attribute '_public_match_client'
Labels: name: area: vector store, name: auto:bug

### System Info

langchain                     0.0.332
Python 3.10.12 
Platform: GCP

### Who can help?

_No response_

### Information

- [ ] The official example notebooks/scripts
- [X] 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

Code to reproduce the problem:
```
from google.cloud import aiplatform
from langchain.embeddings import VertexAIEmbeddings
from langchain.vectorstores.matching_engine import MatchingEngine

embeddings = VertexAIEmbeddings()
embeddings

vector_store = MatchingEngine.from_components(
    index_id=INDEX,
    region=REGION,
    embedding=embeddings,
    project_id=PROJECT_ID,
    endpoint_id=ENDPOINT,
    gcs_bucket_name=DOCS_EMBEDDING)

vector_store.similarity_search('hello world', k=8)
```

Traceback:
```Traceback (most recent call last):
  File "/opt/conda/envs/classifier/lib/python3.10/site-packages/IPython/core/interactiveshell.py", line 3526, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-13-c6115207e7f5>", line 1, in <module>
    relevant_documentation = vector_store.similarity_search('hello world', k=8)
  File "/opt/conda/envs/classifier/lib/python3.10/site-packages/langchain/vectorstores/matching_engine.py", line 291, in similarity_search
    docs_and_scores = self.similarity_search_with_score(
  File "/opt/conda/envs/classifier/lib/python3.10/site-packages/langchain/vectorstores/matching_engine.py", line 202, in similarity_search_with_score
    return self.similarity_search_by_vector_with_score(
  File "/opt/conda/envs/classifier/lib/python3.10/site-packages/langchain/vectorstores/matching_engine.py", line 234, in similarity_search_by_vector_with_score
    if self.endpoint._public_match_client:
AttributeError: 'MatchingEngineIndexEndpoint' object has no attribute '_public_match_client'
```

**Expected Behavior**: 
No Error

**Analysis**:
The newest changes in https://github.com/langchain-ai/langchain/pull/10056 added the following to matching engine. [Source](https://github.com/langchain-ai/langchain/blob/869df62736f9084864ab907e7ec5736dd19f05d4/libs/langchain/langchain/vectorstores/matching_engine.py#L234)

`if self.endpoint._public_match_client:`

However, in GCP's MatchingEngineIndexEndpoint Class, object _public_match_client does not get instantiated until the following check passes. [Source](https://github.com/googleapis/python-aiplatform/blob/fcf05cb6da15c83e91e6ce5f20ab3e6649983685/google/cloud/aiplatform/matching_engine/matching_engine_index_endpoint.py#L132-L133)

```
        if self.public_endpoint_domain_name:
            self._public_match_client = self._instantiate_public_match_client()
``` 

Therefore I think the if self.endpoint._public_match_client check may be preventing all Private Network users from using vector search with matching Engine.