[Swin] Add Swin SimMIM checkpoints
# What does this PR do?

This PR adds 2 checkpoints for Swin Transformer pre-trained using the SimMIM objective (taken from [here](https://github.com/microsoft/Swin-Transformer/blob/main/MODELHUB.md#simmim-pretrained-swin-v1-models)).

They are on the hub: https://huggingface.co/models?other=simmim

It also fixes an important bug in `modeling_swin.py` regarding the window size not being set properly.
_The documentation is not available anymore as the PR was closed or merged._