added n_threads functionality for gpt4all (#5427)

# Added support for modifying the number of threads in the GPT4All model

I have added the capability to modify the number of threads used by the
GPT4All model. This allows users to adjust the model's parallel
processing capabilities based on their specific requirements.

## Changes Made
- Updated the `validate_environment` method to set the number of threads
for the GPT4All model using the `values["n_threads"]` parameter from the
`GPT4All` class constructor.

## Context
Useful in scenarios where users want to optimize the model's performance
by leveraging multi-threading capabilities.
Please note that the `n_threads` parameter was included in the `GPT4All`
class constructor but was previously unused. This change ensures that
the specified number of threads is utilized by the model .

## 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 testing is not required.

---------

Co-authored-by: Dev 2049 <dev.dev2049@gmail.com>

langchain/llms/gpt4all.py

@@ -131,24 +131,27 @@ class GPT4All(LLM):
         """Validate that the python package exists in the environment."""
         try:
             from gpt4all import GPT4All as GPT4AllModel
-
-            full_path = values["model"]
-            model_path, delimiter, model_name = full_path.rpartition("/")
-            model_path += delimiter
-
-            values["client"] = GPT4AllModel(
-                model_name=model_name,
-                model_path=model_path or None,
-                model_type=values["backend"],
-                allow_download=False,
-            )
-            values["backend"] = values["client"].model.model_type
-
         except ImportError:
-            raise ValueError(
+            raise ImportError(
                 "Could not import gpt4all python package. "
                 "Please install it with `pip install gpt4all`."
             )
+
+        full_path = values["model"]
+        model_path, delimiter, model_name = full_path.rpartition("/")
+        model_path += delimiter
+
+        values["client"] = GPT4AllModel(
+            model_name,
+            model_path=model_path or None,
+            model_type=values["backend"],
+            allow_download=False,
+        )
+        if values["n_threads"] is not None:
+            # set n_threads
+            values["client"].model.set_thread_count(values["n_threads"])
+        values["backend"] = values["client"].model.model_type
+
         return values
 
     @property