[CLIP] Add padding embeddings
# What does this PR do?

This PR adds optional per-token padding embeddings (not the usual kind of padding, these are closer to postion embeddings in implementation).

These are needed to convert GLIDE text encoders: https://github.com/openai/glide-text2im/blob/18bf97a07446874693263715af6a73a09cbe81e2/glide_text2im/text2im_model.py#L66

_The documentation is not available anymore as the PR was closed or merged._<|||||>Nevermind, had to rewrite the whole attention layer for GLIDE anyway, so I'll just add a new model