Add LLaMA Causal LM with 7B presets

[tirthasheshpatel]

This PR adds the LLaMA Causal LM along with a weight conversion script for the 7B presets (LLaMA 7B and LLaMA Chat 7B).

Tested that the outputs of the models match with 1e-4 tolerance on CPU and float32.

TODO:

• [x] Upload the presets on Kaggle.
• [x] Look into why huggingface offers different versions of the Rotary Embeddings layers.
  • Looks like it's just an opt-in if the user wants to experiment or train a model from scratch. Might not be something we want at this stage.
• [ ] Implement fixes size cache and cache update. Most probably, will leave this for a follow-up PR.

[tirthasheshpatel]

Changes on master are breaking the tests in this PR. Will sync and push the final changes.

[mattdangerw]

Thanks!