:sparkles: update image classification example
Author: nateraw
Pull request: yes

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Fixes #13663 
- removed `image_size` arg 
- chose to use feature extractor to define the params of the torchvision transforms
- If you want to override the default of 224, it probably means you're pre-training. You can define a preprocessor config and pass `--feature_extractor_name preprocessor_config.json` if needed.

Fixes #13802 
- Updated torch to be `torch>=1.5.0`, as that was the lowest version that worked for me.


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Comments:
@nateraw,

I went back to the deepspeed model zoo PR from some months back https://github.com/huggingface/transformers/pull/12695
where I used `--image_size=30` to keep the test as light as possible. 224 is pretty heavy. so now these tests break as I still had the old arg.

what's the easiest way to set image_size=30 in the new way?
I don't see this being documented at: https://github.com/huggingface/transformers/tree/master/examples/pytorch/image-classification

You did mention `--feature_extractor_name preprocessor_config.json` but I have no idea what to put in the config file. Could this please be documented in the corresponding README.md of the example?

Thank you!


So what was needed is:

```
$ cat vit_feature_extractor.json
{
    "feature_extractor_type": "ViTFeatureExtractor",
    "size": 30
}

$ ... run_image_classification.py ... --feature_extractor_name vit_feature_extractor.json
```

It'd probably be useful to put in `README.md` as it's far from trivial to figure out from our existing docs.