clip : enable gpu backend (#4205)

* clip: enable CUDA backend

* add missing kernels

* add enough padding for alignment

* remove ggml_repeat of clip.cpp

* add metal backend

* llava : fixes

- avoid ggml_repeat
- use GGML_USE_ instead of CLIP_USE_ macros
- remove unused vars

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>