llava : add support for moondream vision language model (#6899)

* add support for moondream vision language model

This required making the following changes to the CLIP model:

1. Support for patch embedding bias.
2. Make class embedding and pre-layernorm optional.
3. Add support for post-layernorm.

* Update examples/llava/clip.cpp

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