Add GPTNeoX Model

[shivance]

Pythia is a suite of 16 LLMs all trained on public data seen in the exact same order and ranging in size from 70M to 12B parameters. The model was developed with intention to facilitate research in many areas. That's why I think this would be a good addition to KerasNLP. I'll work on adding following as a part of Google Summer of Code

• Backbone Adding GPTNeoXBackbone #1056
• Tokenizer Add GPTNeoXTokenizer #1085
• Preprocessor
• Presets
• Tasks

[innat]

#929