Add additional VertexAI Params (#5837)

## Changes

- Added the `stop` param to the `_VertexAICommon` class so it can be set
at llm initialization

## Example Usage

```python
VertexAI(
    # ...
    temperature=0.15,
    max_output_tokens=128,
    top_p=1,
    top_k=40,
    stop=["\n```"],
)
```

## Possible Reviewers

- @hwchase17 
- @agola11

langchain/llms/vertexai.py

@@ -29,6 +29,8 @@ class _VertexAICommon(BaseModel):
     top_k: int = 40
     "How the model selects tokens for output, the next token is selected from "
     "among the top-k most probable tokens."
+    stop: Optional[List[str]] = None
+    "Optional list of stop words to use when generating."
     project: Optional[str] = None
     "The default GCP project to use when making Vertex API calls."
     location: str = "us-central1"
@@ -48,11 +50,13 @@ class _VertexAICommon(BaseModel):
         }
         return {**base_params}
 
-    def _predict(self, prompt: str, stop: Optional[List[str]]) -> str:
+    def _predict(self, prompt: str, stop: Optional[List[str]] = None) -> str:
         res = self.client.predict(prompt, **self._default_params)
         return self._enforce_stop_words(res.text, stop)
 
-    def _enforce_stop_words(self, text: str, stop: Optional[List[str]]) -> str:
+    def _enforce_stop_words(self, text: str, stop: Optional[List[str]] = None) -> str:
+        if stop is None and self.stop is not None:
+            stop = self.stop
         if stop:
             return enforce_stop_tokens(text, stop)
         return text