Custom RoPE Scaling

[LLukas22]

Closes #378.

Adds custom context scaling to llama, falcon, gpt-j, gpt-neox.

Adds an Option<ggml::CustomRoPEArguments> parameter to the ModelParameters.

Adds the optional --rope-base and --rope-scaling cli parameters.

[philpax]

Code looks good. What's the easiest way to test it?

[LLukas22]

1. Sample command for 8k context of llama 2:
   cargo run --release --features cublas -- infer -a llama -m "C:\Users\lkreu\Downloads\llama-2-13b-chat.ggmlv3.q5_K_M.bin" -p "A llama riding a crab" --use-gpu --rope-scaling 0.5 --num-ctx-tokens 8192 --ignore-eos --stats

2. Sit back and get some coffee☕ (8192 tokens are a lot of tokens to be generated)

16k context is also possible by setting rope-scaling to 0.25 but then i don't have enough VRAM to infer on my GPU.

[LLukas22]

The generated text gets repetitive after some time but i guess that's a smapler/setting issue.
lama_story.txt

[philpax]

Great work! I just tested it with LLongMa-2; it's a bit finicky, but that shouldn't be a problem from us. I've revised the names a little to match llama.cpp / refer to frequency, but the rest is the same. Will merge once CI passes 🚀