Add 'download_dir' argument to VLLM (#9754)

- Description:
Add a 'download_dir' argument to VLLM model (to change the cache
download directotu when retrieving a model from HF hub)
- Issue:
On some remote machine, I want the cache dir to be in a volume where I
have space (models are heavy nowadays). Sometimes the default HF cache
dir might not be what we want.
- Dependencies:
None

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

libs/langchain/langchain/llms/vllm.py

@@ -62,6 +62,10 @@ class VLLM(BaseLLM):
     dtype: str = "auto"
     """The data type for the model weights and activations."""
 
+    download_dir: Optional[str] = None
+    """Directory to download and load the weights. (Default to the default 
+    cache dir of huggingface)"""
+
     vllm_kwargs: Dict[str, Any] = Field(default_factory=dict)
     """Holds any model parameters valid for `vllm.LLM` call not explicitly specified."""
 
@@ -84,6 +88,7 @@ class VLLM(BaseLLM):
             tensor_parallel_size=values["tensor_parallel_size"],
             trust_remote_code=values["trust_remote_code"],
             dtype=values["dtype"],
+            download_dir=values["download_dir"],
             **values["vllm_kwargs"],
         )