Improve Swin for VisionEncoderDecoder
# What does this PR do?

This PR improves support for the Swin Transformer with the `VisionEncoderDecoderModel` framework.

Thanks to @FrancescoSaverioZuppichini we now have general formulas to compute the sequence length and hidden size of the last hidden state of Swin Transformer.


_The documentation is not available anymore as the PR was closed or merged._