:sparkles: update image classification example
# What does this PR do?

<!--
Congratulations! You've made it this far! You're not quite done yet though.

Once merged, your PR is going to appear in the release notes with the title you set, so make sure it's a great title that fully reflects the extent of your awesome contribution.

Then, please replace this with a description of the change and which issue is fixed (if applicable). Please also include relevant motivation and context. List any dependencies (if any) that are required for this change.

Once you're done, someone will review your PR shortly (see the section "Who can review?" below to tag some potential reviewers). They may suggest changes to make the code even better. If no one reviewed your PR after a week has passed, don't hesitate to post a new comment @-mentioning the same persons---sometimes notifications get lost.
-->

<!-- Remove if not applicable -->

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.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

<!-- Your PR will be replied to more quickly if you can figure out the right person to tag with @

 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 Please tag fewer than 3 people.

Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

 -->

@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.
