Add LLaMa v2 to KerasNLP

[kanpuriyanawab]

https://twitter.com/ylecun/status/1681336284453781505

I'm up for contributing this.

[kanpuriyanawab]

Putting roadmap

• Llama Backbone
• Tokenizer and Preprocessor
• Tasks
• Presets

[sahusiddharth]

@shivance

I would like to work on this

[mattdangerw]

This is done!