Support HF LLaMA ckpt conversion

[lsy323]

Added --from_hf option in convert_checkpoint.py for HF checkpoint. Only LLaMA is supported now. Quantization conversion is not supported with HF checkpoint.

Enable converting HF llama checkpoint by

python -m convert_checkpoints --model_name=llama-2 \
    --input_checkpoint_dir=$input_ckpt_dir \
    --output_checkpoint_dir=$output_ckpt_dir \
    --from_hf=True

The guide to add support for HF checkpoint will be done in a following PR.

Only tested with HF 7B model, 70B not tested yet