AttributeError: 'GPT4All' object has no attribute 'model_type' (langchain 0.0.190)

### System Info

Hi, this is related to #5651 but (on my machine ;) ) the issue is still there.

## Versions

* Intel Mac with latest OSX
* Python 3.11.2
* langchain 0.0.190, includes fix for #5651 
* ggml-mpt-7b-instruct.bin, downloaded at June 5th from https://gpt4all.io/models/ggml-mpt-7b-instruct.bin

### Who can help?

@pakcheera @bwv988 First of all: thanks for the report and the fix :). Did this issues disappear on you machines?

### Information

- [ ] The official example notebooks/scripts
- [X] My own modified scripts

### Related Components

- [X] LLMs/Chat Models
- [ ] Embedding Models
- [ ] Prompts / Prompt Templates / Prompt Selectors
- [ ] Output Parsers
- [ ] Document Loaders
- [ ] Vector Stores / Retrievers
- [ ] Memory
- [ ] Agents / Agent Executors
- [ ] Tools / Toolkits
- [ ] Chains
- [ ] Callbacks/Tracing
- [ ] Async

### Reproduction


## Error message

```shell
╭─────────────────────────────── Traceback (most recent call last) ────────────────────────────────╮
│ /Users/christoph/src/sandstorm-labs/development-tools/ai-support-chat/gpt4all/chat.py:30 in      │
│ <module>                                                                                         │
│                                                                                                  │
│   27 │   model_name="all-mpnet-base-v2")                                                         │
│   28                                                                                             │
│   29 # see https://gpt4all.io/models/ggml-mpt-7b-instruct.bin                                    │
│ ❱ 30 llm = GPT4All(                                                                              │
│   31 │   model="./ggml-mpt-7b-instruct.bin",                                                     │
│   32 │   #backend='gptj',                                                                        │
│   33 │   top_p=0.5,                                                                              │
│                                                                                                  │
│ in pydantic.main.BaseModel.__init__:339                                                          │
│                                                                                                  │
│ in pydantic.main.validate_model:1102                                                             │
│                                                                                                  │
│ /Users/christoph/src/sandstorm-labs/development-tools/ai-support-chat/gpt4all/venv/lib/python3.1 │
│ 1/site-packages/langchain/llms/gpt4all.py:156 in validate_environment                            │
│                                                                                                  │
│   153 │   │   if values["n_threads"] is not None:                                                │
│   154 │   │   │   # set n_threads                                                                │
│   155 │   │   │   values["client"].model.set_thread_count(values["n_threads"])                   │
│ ❱ 156 │   │   values["backend"] = values["client"].model_type                                    │
│   157 │   │                                                                                      │
│   158 │   │   return values                                                                      │
│   159                                                                                            │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯
```

As you can see in _gpt4all.py:156_ contains the changed from the fix of #5651

## Code

```python
from langchain.llms import GPT4All

# see https://gpt4all.io/models/ggml-mpt-7b-instruct.bin
llm = GPT4All(
    model="./ggml-mpt-7b-instruct.bin",
    #backend='gptj',
    top_p=0.5,
    top_k=0,
    temp=0.1,
    repeat_penalty=0.8,
    n_threads=12,
    n_batch=16,
    n_ctx=2048)
```
FYI I am following [this example in a blog post](https://dev.to/akshayballal/beyond-openai-harnessing-open-source-models-to-create-your-personalized-ai-companion-1npb).

### Expected behavior

I expect an instance of _GPT4All_ instead of a stacktrace.