Instead of MatVecLoop, we use MatVec and we combine k and v
into one 2 * kQKVDim long vector so that K and V projections
can be combined into one MatVec operation.

Benchmark results (summarization with 1600 tokens for prefill
and essay writing with 500 tokens for generation):

```
                   Prefill speed                Generation speed
Num threads      BEFORE       AFTER            BEFORE       AFTER
4                 9.81 t/s     9.96 t/s       8.39 t/s     8.46 t/s
18               31.50 t/s    36.67 t/s      23.10 t/s    25.83 t/s
32               45.36 t/s    58.91 t/s      27.60 t/s    31.25 t/s
64               57.72 t/s    80.64 t/s      35.40 t/s    39.76 t/s
```