Add FlaxConvNext
# What does this PR do?
Adds a Flax ConvNext model to transformers.

## Who can review?
This is currently an in-progress PR as discussed with @patil-suraj 
TODO: Fix docstrings, ensure proper parameter name mapping for inter-framework weight conversion and write tests
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15977). All of your documentation changes will be reflected on that endpoint.<|||||>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.<|||||>Hey @DarshanDeshpande ! Sorry to only reply here now. This slipped through the cracks. LMK if you are still interested to add this model in Flax :)<|||||>Hey @patil-suraj, I am still interested in adding the Flax ConvNeXt but I will need some guidance with the TODOs mentioned in the first commit.<|||||>I will go through the PR tomorrow and do an initial review.<|||||>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.