[Idefics3] Move image features to same device as input embeds
Author: shyshin
Pull request: yes

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Fixes [#35031](https://github.com/huggingface/transformers/issues/35031) 
Summary: 
Got a device mismatch issue on running `trainer.train()`.
```
File ~/.local/lib/python3.11/site-packages/transformers/models/idefics3/modeling_idefics3.py:904, in Idefics3Model.inputs_merger(self, input_ids, inputs_embeds, image_hidden_states)
    902 # cast to the dtype of the input_embeds to support quantized models
    903 reshaped_image_hidden_states = reshaped_image_hidden_states.to(inputs_embeds.dtype)
--> 904 new_inputs_embeds[special_image_token_mask] = reshaped_image_hidden_states
    905 return new_inputs_embeds

RuntimeError: Expected all tensors to be on the same device, but found at least two devices, cuda:0 and cuda:1
```

Moved `image_features` to same device as `input_embeds`.

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Comments:
You want me to have a change like this?
```
reshaped_image_hidden_states = reshaped_image_hidden_states.to(inputs_embeds.dtype).to(self.device)
new_inputs_embeds[special_image_token_mask] = reshaped_image_hidden_states
```

I will try to run this on my multi device setup.

@shyshin you can do in one go as `reshaped_image_hidden_states = reshaped_image_hidden_states.to(device=self.device, dtype=inputs_embeds.dtype)` :)

@zucchini-nlp  I have updated the code with your suggestions

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_35100). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Seems like a tiny fix without need for core maintainer's approval, so maybe another review from @qubvel  and let's merge

Hey @shyshin ! Would you be able to incorporate the suggested changes ?  If you are busy, I can push the changes and merge the PR :)

@qubvel @zucchini-nlp I am sorry I am really swamped with work atm. It would be really nice if you could push the changes and merge the PR :)