Add ConvNeXT
# What does this PR do?

This PR adds [ConvNeXT](https://github.com/facebookresearch/ConvNeXt) to the library, a convnet inspired by Transformers.

To do:

- [x] rename nielsr to facebook to fix tests
- [x] remove inference.py script
- [x] remove PushToHubMixin hack from feature extractor
- [x] fix "gamma" in `from_pretrained` method. Right now, if a model includes a parameter named "gamma", they are not initialized from the hub, due to [this line](https://github.com/huggingface/transformers/blob/833635e25997a00117076c50bde5c45f9b883ada/src/transformers/modeling_utils.py#L1490).
- [x] discuss model output classes. It feels a bit weird to talk about "hidden states" for convolutional models, most people use the term "features" or "feature maps". These are of shape (batch_size, num_channels, a certain height, a certain width) at each stage, rather than (batch_size, seq_len, hidden_size) at each layer of a Transformer. For now, they are still called `hidden_states` in this PR.
_The documentation is not available anymore as the PR was closed or merged._