Add allow_download as attribute for GPT4All (#5512)

# Added support for download GPT4All model if does not exist

I've include the class attribute `allow_download` to the GPT4All class.
By default, `allow_download` is set to False.

## Changes Made
- Added a new attribute `allow_download` to the GPT4All class.
- Updated the `validate_environment` method to pass the `allow_download`
parameter to the GPT4All model constructor.

## Context
This change provides more control over model downloading in the GPT4All
class. Previously, if the model file was not found in the cache
directory `~/.cache/gpt4all/`, the package returned error "Failed to
retrieve model (type=value_error)". Now, if `allow_download` is set as
True then it will use GPT4All package to download it . With the addition
of the `allow_download` attribute, users can now choose whether the
wrapper is allowed to download the model or not.

## Dependencies
There are no new dependencies introduced by this change. It only
utilizes existing functionality provided by the GPT4All package.

## Testing
Since this is a minor change to the existing behavior, the existing test
suite for the GPT4All package should cover this scenario

Co-authored-by: Vokturz <victornavarrrokp47@gmail.com>

langchain/llms/gpt4all.py

@@ -92,6 +92,9 @@ class GPT4All(LLM):
     """Leave (n_ctx * context_erase) tokens
     starting from beginning if the context has run out."""
 
+    allow_download: bool = False
+    """If model does not exist in ~/.cache/gpt4all/, download it."""
+
     client: Any = None  #: :meta private:
 
     class Config:
@@ -145,7 +148,7 @@ class GPT4All(LLM):
             model_name,
             model_path=model_path or None,
             model_type=values["backend"],
-            allow_download=False,
+            allow_download=values["allow_download"],
         )
         if values["n_threads"] is not None:
             # set n_threads