Adding Q6_0 (#77)

* Adding q6_0 - basics + AVX2/Zen4 working

* Adding q6_0: CUDA dequantize works, but not mmvq

* Adding q6_0: CUDA mmvq works

* Adding q6_0: CUDA cpy, so Q6_0 can be used for KV-cache

* Add q6_0 to CPU flash attention

Disappointing result: for LlaMA-3.2-1B, q6_0 K- and V-cache
gives about the same PPL as q8_0 K-cache and q4_0 V-cache,
while needing the exact same RAM.
I.e., what was the point?

* q6_0: slightly better kv-cache result

Better than q8_0+q4_0, but not as good as q8_0+iq4_nl

* q6_0: works on ARM_NEON

* q6_0: dequantize works on Metal, but not vector dot product

* q6_0: it now works on Metal

Outperforms q5_0 by a significant margin. E.g.
| model                          |       size |     params | backend    | ngl | threads |          test |              t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | ------: | ------------: | ---------------: |
| llama 8B Q6_0                  |   6.08 GiB |     8.03 B | Metal      | 100 |       4 |         tg128 |     44.02 ± 0.08 |
| llama 8B Q5_0                  |   5.21 GiB |     8.03 B | Metal      | 100 |       4 |         tg128 |     40.13 ± 0.12 |
| llama 8B Q6_0                  |   6.08 GiB |     8.03 B | Metal      | 100 |       4 |         pp512 |    500.55 ± 0.32 |
| llama 8B Q5_0                  |   5.21 GiB |     8.03 B | Metal      | 100 |       4 |         pp512 |    448.02 ± 0.27 |

* q6_0: can now be used for kv-cache on Metal

---------

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

common/common.cpp

@@ -2242,6 +2242,9 @@ static ggml_type kv_cache_type_from_str(const std::string & s) {
     if (s == "q5_1") {
         return GGML_TYPE_Q5_1;
     }
+    if (s == "q6_0") {
+        return GGML_TYPE_Q6_0;
+    }
 
     throw std::runtime_error("Invalid cache type: " + s);
 }