Add VideoMAE
# What does this PR do?

This PR adds [VideoMAE](https://github.com/MCG-NJU/VideoMAE), which extends [ViTMAE](https://huggingface.co/docs/transformers/model_doc/vit_mae) to videos. 

The only difference between VideoMAE and ViT is that you need to replace `nn.Conv2d` by `nn.Conv3d` in the patch embedding class. 😂 

To do:

- [ ] Decide on a name for `VideoMAEFeatureExtractor` (should we keep it, or rename to `VideoMAEProcessor`, `VideoMAEPreprocessor`?)
- [ ] Decide on the input format for video models; currently I've chosen `pixel_values` of shape (batch_size, num_frames, num_channels, height, width). The original implementation uses (B, C, T, H, W)
- [ ] Doc examples + tests
- [x] Incorporate changes of #17731 
- [ ] Make VideoMAEFeatureExtractor robust with return_tensors="np" by default, better tests
@NielsRogge do you have any ETA on this feature? I am developing a video classification fine-tuning framework, would love to use this model if it gets merged into main!

Currently only video model is PerceiverIO, right?<|||||>There seems to remain an issue with the docs:

```
Traceback (most recent call last):
  File "/usr/local/bin/doc-builder", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.8/site-packages/doc_builder/commands/doc_builder_cli.py", line 47, in main
    args.func(args)
  File "/usr/local/lib/python3.8/site-packages/doc_builder/commands/build.py", line 96, in build_command
    build_doc(
  File "/usr/local/lib/python3.8/site-packages/doc_builder/build_doc.py", line 405, in build_doc
    sphinx_refs = check_toc_integrity(doc_folder, output_dir)
  File "/usr/local/lib/python3.8/site-packages/doc_builder/build_doc.py", line 460, in check_toc_integrity
    raise RuntimeError(
RuntimeError: The following files are not present in the table of contents:
- model_doc/videomae
Add them to ../transformers/docs/source/en/_toctree.yml.
```<|||||>@LysandreJik yes I was aware of that, should be fixed now.

Don't merge already please, I'm transferring checkpoints and updating the conversion script.<|||||>_The documentation is not available anymore as the PR was closed or merged._