[WIP] Llama2 Integration

[reshinthadithyan]

The following PR adds Llama2 to architecture to MaxText to continue pretraining or train from scratch.

TODO

• [x] Add Llama2 architecture to llama2.py.
• [ ] Add predefined config for different sizes(7B,13B and 70B).
• [ ] Add conversion script to convert hf weights to orbax checkpoint compatible weights

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