[WIP]Add ViTPose to Transformers
# What does this PR do?
Adds ViTPose to Huggingface/Transformers

Code and weights: https://github.com/ViTAE-Transformer/ViTPose
Paper: https://arxiv.org/abs/2204.12484

Fixes #24915 

## Before submitting
- [X] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
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      to it if that's the case.
- [X] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?

## Who can review?
@amyeroberts 
@amyeroberts weights for ViTPose are in PyTorch checkpoint files, should I convert them into .bin or something else?<|||||>@shauray8 You will want to write a conversion script, which translates the names of the weights in the state dict to their equivalent in transformers. These weights will then be loaded into the transformers model. Then we save the transformers model using `model.save_pretrained(...)` which will save out the weights in the desired format (safetensors) as well as all other necessary files such as the model config. 

For this model, because the encoder is ViT, the translation of these weights will follow a similar pattern to the conversion script here: https://github.com/huggingface/transformers/blob/main/src/transformers/models/vit/convert_vit_timm_to_pytorch.py

Similarly, the encoder structure can be implemented directly by using `#Copied from` statements to copy the ViT architecture in the modeling file. 

If you haven't seen it already, I'd also suggest looking through [this doc](https://huggingface.co/docs/transformers/add_new_model) as it covers in detail all the steps for adding a model to transformers. <|||||>Thank you for the information and the link to the conversion script for ViT.