Add CLIPSeg
# What does this PR do?

This PR adds CLIPSeg, a nice extension of CLIP for zero-shot and one-shot (image-guided) image segmentation.

To do:

- [x] transfer checkpoints and update code
- [x] update base_model_prefix
_The documentation is not available anymore as the PR was closed or merged._<|||||>Is the CLIPSeg yet to be released in the latest version?
