Integrating R3M Models into Transformers
# What does this PR do?

This PR integrates the new R3M visual encoder model into transformers. See this [issue](https://github.com/huggingface/transformers/issues/16403).

In its current state, the new model/config can be loaded, trained, and pre-trained models located [here](https://huggingface.co/surajnair) can be loaded using `AutoModel.from_pretrained` and behave as expected.

However, the model is simply a visual encoder on a ResNet backbone, taking a batch of images passing them through the pre-trained ResNet18,34,50, and returning a batch of embeddings. Therefore most of the testing infrastructure fails (as this is not an NLP model). There are also probably boilerplate versions of the model (ex: CausalLM) which are not applicable.

<!-- Remove if not applicable -->

Fixes # 16403


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [X] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ X] 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.
- [ 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? - still a WIP.


## 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.

@edbeeching @LysandreJik  have been helping me so far.

Thanks a lot for your contribution!

@NielsRogge, could you do a first pass on this model?<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.