faster forward following what is done for images
# What does this PR do?
Follow up of #21897

Keeps batch_size active, but restricts it to the imagepart.
Calculates candidate_labels features only once (there's no way with this current pipeline to send different candidate_labels so we can take the optimization.
Ultra narrowed for CLIP but the pipeline has existed for 4 months now with no new models. Given the popularity of CLIP for diffusion models, I think this is ok to overspecify. We can always relax later when new models come in.
This allows to downgrade from ChunkPipeline to regular PIpeline
_The documentation is not available anymore as the PR was closed or merged._<|||||>Failure seems unrelated