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
```
We only used inner_pool in the prefill FFW function, and there we
can achieve sufficient parallelism on the rows of the matrix-vector
multiplications.
Benchmark results on a 1600-token summarization task:
```
Prefill speed
Num threads BEFORE AFTER
4 9.24 t/s 9.76 t/s
18 31.41 t/s 31.16 t/s
32 31.41 t/s 45.13 t/s
64 31.03 t/s 57.85 t/s
```
Move Path into io.h and use for opening files.
Removes dependency of gemma_lib on args.
Separate Windows codepath instead of emulating POSIX functions.
Plus lint fixes.
PiperOrigin-RevId: 626279004
Apoorv Reddy
Zoltan Szabadka
Jan Wassenberg
Andrey Mikhaylov
RangerUFO