Addresses GPT4All wrapper model_type attribute issues #5720. (#5743)

Fixes #5720.

A more in-depth discussion is in my comment here:
https://github.com/hwchase17/langchain/issues/5720#issuecomment-1577047018

In a nutshell, there has been a subtle change in the latest version of
GPT4Alls Python bindings. The change I submitted yesterday is compatible
with this version, however, this version is as of yet unreleased and
thus the code change breaks Langchain's wrapper under the currently
released version of GPT4All.

This pull request proposes a backwards-compatible solution.

langchain/llms/gpt4all.py

@@ -153,7 +153,12 @@ class GPT4All(LLM):
         if values["n_threads"] is not None:
             # set n_threads
             values["client"].model.set_thread_count(values["n_threads"])
-        values["backend"] = values["client"].model_type
+
+        try:
+            values["backend"] = values["client"].model_type
+        except AttributeError:
+            # The below is for compatibility with GPT4All Python bindings <= 0.2.3.
+            values["backend"] = values["client"].model.model_type
 
         return values