Remove extra Dot() overload
MatVecAdd always adds, use MatVecT<kAdd> if conditional.
Remove ununsed MatVecAddLoop and MatVecLoop
No longer tsan-verify even_odd
PiperOrigin-RevId: 631377279
We compute all three projections with one MatVec and then copy
the kv part to the cache.
Benchmark results for 7b-it model that uses MHA blocks (summarization with
1600 tokens for prefill and essay writing with 500 tokens for generation):
```
Prefill speed Generation speed
Num threads BEFORE AFTER BEFORE AFTER
32 13.75 t/s 14.80 t/s 9.22 t/s 9.77 t/s
64 19.89 t/s 24.83 t/s 12.46 t/s 13.66 t/s
```
Jan Wassenberg
Phil Culliton
Zoltan Szabadka