Add XCiT Model
### Model description

Cross-covariance image transformer (XCiT) is built upon XCA. It combines the accuracy of conventional transformers with the scalability of convolutional architectures. [Paper](https://arxiv.org/abs/2106.09681)

### Open source status

- [X] The model implementation is available
- [X] The model weights are available

### Provide useful links for the implementation

Official Implementation: https://github.com/facebookresearch/xcit
Timm Implementation: https://github.com/rwightman/pytorch-image-models/blob/main/timm/models/xcit.py