IQ4_NL_X4 (#118)

* Adding iq4_nl_x4

Looks very promising - I get PP-512(LLaMA-3.1-8B) = 230 t/s
on the Ryzen-7950X! This is faster than any other quant and
~40% faster than iq4_nl.

* iq4_nl_x4: getting amazing

This Zen4 variant gets us to PP-512(LLaMA-3.1-8B) = 263 t/s!

* iq4_nl_x4: AVX2

Here we gain only 25% compared to iq4_nl

* iq4_nl_x4: NEON

On M2-Max we get PP-512(LLaMA-3.1-8B) = 109.7 t/s, up from
82.4 t/s for iq4_nl.

* iq4_nl_x4: minor NEON improvement and cleanup

This gets us to 110.3 t/s. In comparison,
IQ4_NL_4_4 in mainline llama.cpp achieves 92.3 t/s.

* iq4_nl_x4: NEON specialization for matrix x vector

---------

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

include/llama.h

@@ -179,6 +179,8 @@ extern "C" {
         LLAMA_FTYPE_MOSTLY_IQ3_KL        = 146, // except 1d tensors
         LLAMA_FTYPE_MOSTLY_IQ2_KS        = 147, // except 1d tensors
         LLAMA_FTYPE_MOSTLY_IQ4_KSS       = 148, // except 1d tensors
+                                                //
+        LLAMA_FTYPE_MOSTLY_IQ4_NL_X4     = 225, // except 1d tensors
 
         LLAMA_FTYPE_GUESSED = 1024, // not specified in the model file
     };