feat: perplexity.

[b4rtaz]

This pull request adds a new option to dllama.

./dllama perplexity --prompt "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Suspendisse consectetur nibh risus, id volutpat justo pulvinar sed. Nulla gravida justo id mauris ullamcorper suscipit. Sed ultrices dui ac justo consequat, vel bibendum orci dictum. Fusce nisi turpis, blandit eu porttitor ac, tincidunt in augue." \
  --model models/llama3_1_8b_instruct_q40/dllama_model_llama3_1_8b_instruct_q40.m --tokenizer models/llama3_1_8b_instruct_q40/dllama_tokenizer_llama3_1_8b_instruct_q40.t --buffer-float-type q80 --nthreads 4 --max-seq-len 4096