add VisionTextDualEncoder and CLIP fine-tuning script
# What does this PR do?

This PR adds a fine-tuning script for `VisionTextDualEncoder` and `CLIP` model.  

The script is named `run_clip.py` since clip also refers to the training scheme (contrastive-language-image) and not just the model.  But feel free to suggest other names.

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