llama : add Q3_K_XS (#5060)

* Add Q3_K_XS - intermediate size between Q2_K and Q3_K_S

* Q3_K_XS: quanize first 1/8 of ffn_down layers with Q4_K

Together with an importance matrix, this brings perplexity
for LLaMA-v2-70B below the perplexity of the former Q2_K
with a 800 MB smaller quantized model size.

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Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>