This commit introduces a CPU reference implementation for GGML,
designed primarily for testing and validation purposes.
The motivation for this addition is to have a pure C CPU backend
implementation that does not use any hardware-specific optimizations
or intrinsics. This will allow for testing the CPU backend variants
against the reference implementation to ensure correctness
Building:
```console
$ cmake -B build \
-DGGML_CPU_REF_BACKEND=ON
-DGGML_BACKEND_DL=ON \
-DGGML_CPU_ALL_VARIANTS=ON
```
List availble cpu architectures/variants:
```console
$ ./build/bin/test-backend-ops cpu-variants --list
CPU variants:
CPU-haswell - 12th Gen Intel(R) Core(TM) i7-1260P
CPU-sse42 - 12th Gen Intel(R) Core(TM) i7-1260P
CPU-x64 - 12th Gen Intel(R) Core(TM) i7-1260P
CPU-alderlake - 12th Gen Intel(R) Core(TM) i7-1260P
CPU-sandybridge - 12th Gen Intel(R) Core(TM) i7-1260P
```
Run tests:
```console
./build-ref/bin/test-backend-ops cpu-variants --variant CPU-alderlake -o ADD
CPU-ref features:
SSE2 = 1
CPU-alderlake features:
SSE2 = 1
SSE3 = 1
SSSE3 = 1
AVX = 1
AVX_VNNI = 1
AVX2 = 1
F16C = 1
FMA = 1
BMI2 = 1
LLAMAFILE = 1
OPENMP = 1
REPACK = 1
Testing CPU variant 'CPU-alderlake' against 'CPU-ref' backend...
ADD(type=f16,ne=[1,1,8,1],nr=[1,1,1,1],nf=1): OK
ADD(type=f16,ne=[1,1,1,1],nr=[32,1,1,1],nf=1): OK
...
```
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