Image column name missing for CLIP
# What does this PR do?

The patch makes sure that the image features do not get removed. Otherwise  **transform_images(examples)** will have a **KeyError** for the image_column when trying to transform the image.

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Fixes # (issue)


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The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23638). All of your documentation changes will be reflected on that endpoint.<|||||>Hi @TJKlein, thanks for opening this PR. 

Could you provide a bit more information about the issue this is resolving? There isn't a `transform_images` method in the `run_clip.py` script or in the [dataset preparation script](https://huggingface.co/datasets/ydshieh/coco_dataset_script/blob/main/coco_dataset_script.py)<|||||>Well, there is:
https://github.com/huggingface/transformers/blob/28f589f0a46cced297fba46014ee73b862fa247b/examples/pytorch/contrastive-image-text/run_clip.py#L396

https://github.com/huggingface/transformers/blob/28f589f0a46cced297fba46014ee73b862fa247b/examples/pytorch/contrastive-image-text/run_clip.py#L410

Since the image_column is not a standard feature it will get pruned from trainer. Therefore it has to be specified to the trainer. That's what I did.

```
trainer._signature_columns=["input_ids", "attention_mask", data_args.image_column]
```<|||||>@TJKlein Apologies, I completely missed the function, sorry! 

Could you expand on the issue this tackles - perhaps with a code snippet to reproduce? I'm able to run the example script with the following command: 

```python
python examples/pytorch/contrastive-image-text/run_clip.py \
    --output_dir ./clip-roberta-finetuned \
    --model_name_or_path ../clip-roberta \
    --data_dir $PWD/data \
    --dataset_name ydshieh/coco_dataset_script \
    --dataset_config_name=2017 \
    --image_column image_path \
    --caption_column caption \
    --remove_unused_columns=False \
    --do_train  --do_eval \
    --per_device_train_batch_size="64" \
    --per_device_eval_batch_size="64" \
    --learning_rate="5e-5" --warmup_steps="0" --weight_decay 0.1 \
    --overwrite_output_dir
```

`_signature_columns` is a private attribute, and not something we want to modify directly like this. Understanding a bit more about how the issue arises, hopefully we'll be able to find another approach.  <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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