✨ Add PyTorch image classification example
Author: nateraw
Pull request: yes

# What does this PR do?

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Adds PyTorch image classification example. For now, it uses `torchvision.datasets.ImageFolder` to load local image folders (just like the flax image classification example). In the future, we will switch to using the `datasets` package's image folder (once it exists). 

Marking as draft for now as I'm still working through cleaning up changes I made from [this example](https://github.com/nateraw/vision/tree/main/image-classification) I wrote earlier that uses `datasets` instead.

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
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Comments:
Nice!! Relevant for #13080 

I'll review this PR in detail (thanks for working on this!). Regarding the fixtures for the tests, I've recently moved these files to the hf-internal-testing organization on the [hub](https://huggingface.co/hf-internal-testing). This makes it more clear, as otherwise these fixture files are also downloaded when people do a `git clone` of the library.

Last nit on my side: can we move the vision folder to be `image-classification`? We will have other kinds of vision examples in the future.

Ok, addressed most of the comments. Merging as-is for now.

@NielsRogge I did not address these two items, however I can in future PRs (if need be):

- Adding test data to `datasets` library. 
- Adjusting train/validation/test split logic. 

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