[docs] update `load_tool` 
Author: faaany
Pull request: yes

## What does this PR do?
This model `m-ric/text-to-image` seems to not exit anymore on the HuggingFace Model Hub. When running the example code, I got the following error: 
```bash
You're loading a tool from the Hub from None. Please make sure this is a source that you trust as the code within that tool will be executed on your machine. Always verify the code of the tools that you load. We recommend specifying a `revision` to ensure you're loading the code that you have checked.
Traceback (most recent call last):
  File "/home/sdp/fanli/transformers/training.py", line 10, in <module>
    image_generation_tool = load_tool("m-ric/text-to-image")
  File "/home/sdp/fanli/transformers/src/transformers/agents/tools.py", line 801, in load_tool
    return Tool.from_hub(task_or_repo_id, model_repo_id=model_repo_id, token=token, **kwargs)
  File "/home/sdp/fanli/transformers/src/transformers/agents/tools.py", line 357, in from_hub
    return tool_class(**kwargs)
  File "/home/sdp/fanli/transformers/src/transformers/agents/tools.py", line 97, in new_init
    self.validate_arguments(do_validate_forward=do_validate_forward)
  File "/home/sdp/fanli/transformers/src/transformers/agents/tools.py", line 152, in validate_arguments
    raise TypeError(f"You must set an attribute {attr} of type {expected_type.__name__}.")
TypeError: You must set an attribute inputs of type Dict.
```
I only found [this ](https://huggingface.co/spaces/m-ric/text-to-image/blob/main/tool.py) page, so I used the code from there to update the doc. If it is better to use load tool directly from model hub, pls advice me one, so I can update my PR to use a working `image_generation_tool`. 

@stevhliu


Comments:
Hi! 👋 

I run into the same problem since it is also happening in the [cookbooks](https://huggingface.co/learn/cookbook/agents).
This code (the cookbook) was previously working and the tool is actually [available](https://huggingface.co/spaces/m-ric/text-to-image/blob/main/tool.py) and wasn't removed 😄
I believe that the problem comes from the type check done in [tools.py](https://github.com/huggingface/transformers/blob/25f510a9c690f195032eb0047076fcbec84ec961/src/transformers/agents/tools.py#L144):

```
required_attributes = {
    "description": str,
    "name": str,
    "inputs": Dict,
    "output_type": str,
}
authorized_types = ["string", "integer", "number", "image", "audio", "any", "boolean"]

for attr, expected_type in required_attributes.items():
    attr_value = getattr(self, attr, None)
    if not isinstance(attr_value, expected_type):
        raise TypeError(f"You must set an attribute {attr} of type {expected_type.__name__}.")
```

As stated, it expects a `Dict` for the inputs instead of a `dict`.

A possible solution would be updating the [m-ric/text-to-image](https://huggingface.co/spaces/m-ric/text-to-image/blob/main/tool.py) using `Dict`. Another possible solution would be changing the [validate_arguments](https://github.com/huggingface/transformers/blob/25f510a9c690f195032eb0047076fcbec84ec961/src/transformers/agents/tools.py#L140) to be more flexible. 

I could implement any of them if help is needed 😄 



Experimenting further, I've found that when loading a tool from the hub, the method [from_hub](https://github.com/huggingface/transformers/blob/25f510a9c690f195032eb0047076fcbec84ec961/src/transformers/agents/tools.py#L248C9-L248C17) is called and in this line:

https://github.com/huggingface/transformers/blob/25f510a9c690f195032eb0047076fcbec84ec961/src/transformers/agents/tools.py#L352

The inner code is executed: 

https://github.com/huggingface/transformers/blob/25f510a9c690f195032eb0047076fcbec84ec961/src/transformers/agents/tools.py#L353

The first value `tool_class.inputs` is a `dict`, which is correct whereas the second value `custom_tool["inputs"]` is an string. Essentially, this causes the issue.


````
# tool_class.inputs
<class 'dict'>
{'prompt': {'type': 'string', 'description': "The image generator prompt. Don't hesitate to add details in the prompt to make the image look better, like 'high-res, photorealistic', etc."}}

# custom_tool["inputs"]
<class 'str'>
{'prompt': {'type': 'text', 'description': "The image generator prompt. Don't hesitate to add details in the prompt to make the image look better, like 'high-res, photorealistic', etc."}}

````
Commenting out these two lines, the code runs correctly without any issue.

cc @aymeric-roucher 😄 

What do you think @stevhliu?

For the other example in  [Agents, supercharged - Multi-agents, External tools, and more](https://huggingface.co/docs/transformers/agents_advanced) `model_download_tool = load_tool("m-ric/hf-model-downloads")`. The result is similar in the same place:

```
# tool_class.inputs
<class 'dict'>
{'task': {'type': <class 'str'>, 'description': 'the task category (such as text-classification, depth-estimation, etc)'}}

# custom_tool["inputs"]
<class 'str'>
{'task': {'type': <class 'str'>, 'description': 'the task category (such as text-classification, depth-estimation, etc)'}}
```

In order to make it work, in addition to commenting out the previously mentioned lines, you must also add in that same place the following:

```
tool_class.output_type = 'string'
```

I think that the issue comes from:

* `custom_tool["inputs"]` which is not correctly converted to a dict
* The custom Tool definition may have changed a bit since [HFModelDownloadsTool](https://huggingface.co/spaces/m-ric/hf-model-downloads/blob/main/model_downloads.py) and [TextToImageTool](https://huggingface.co/spaces/m-ric/text-to-image/blob/main/tool.py) differ a little bit.


I'm happy to review it further and generate a PR if needed 😄 

Wow thanks so much for your investigation @sergiopaniego! I'm in favor of commenting out those two lines, but let's wait and see what @aymeric-roucher thinks 🙂 

Well spotted @faaany and @sergiopaniego ! This is indeed due to the assignment of tool_class.inputs.

I've made a fix for this input type error just a few hours ago while adding new stuff in [this PR](https://github.com/huggingface/transformers/pull/34549) (merged now). So your issue should be fixed now!

And I've manually fixed the tools, all should work now. The rendering for https://huggingface.co/spaces/m-ric/hf-model-downloads still fails in the Space (you can still pull the tool and it works properly) but will be merged in a PR very soon!

Hi all, thanks so much for the discussion! Closing this PR, as it got fixed in @aymeric-roucher 's PR.