[ViTMAE] Add image pretraining script
# What does this PR do?

This PR adds an example script to a new folder, "image-pretraining". It allows the user to pre-train a Vision Transformer (ViT) using the [MAE](https://arxiv.org/abs/2111.06377) method. More specifically, one can pre-train a `ViTMAEForPreTraining` with the script. After pre-training, one can easily load the weights into a `ViTForImageClassification`, obtaining SOTA accuracy. 🥇 

Some things that require some feedback:

- regarding providing a custom dataset: this should be updated in the README. The original script uses torchvision's handy ImageFolder as seen [here](https://github.com/facebookresearch/mae/blob/6a2ba402291005b003a70e99f7c87d1a2c376b0d/main_pretrain.py#L128). Can we have something similar with `datasets`? ([#2830](https://github.com/huggingface/datasets/pull/2830))
- hyperparameters: here are the ones used in the original paper:
<img width="521" alt="Schermafbeelding 2022-01-20 om 10 22 07" src="https://user-images.githubusercontent.com/48327001/150310271-5d3afc1b-43a9-46be-bc9b-d7a53c9c65de.png">

=> I'd like to mimic these as closely as possible.
_The documentation is not available anymore as the PR was closed or merged._<|||||>We do have a speech-pretraining folder in the examples directory, which also only works for a single model (`Wav2Vec2ForPreTraining`).<|||||>Do what you want then :-) 