ggml : add ggml_scale_bias

[ngxson]

Ref discussion: #14400 (comment)

Added ggml_scale_bias(ctx, a, s, b) in this PR, which allows calculating x = a * s + b

I only added Metal kernel for now, just for discussion. @ggerganov does this looks good to you?

TODO: support other backends

[ngxson]

I hope this won't have a significant impact on the performance

[ggerganov]

Think it's a useful extension of the operator.

[ggerganov]

Merge these in ggml_vec_mad1_f32(). If you want, you can try to add a GGML_SIMD version using GGML_F32_VEC_FMA - it's quite simple. But also can leave it a basic for loop without SIMD.

[ehoogeveen-medweb]

Quick question: Is the "scale-bias" nomenclature more appropriate here than "multiply-add"? From an outsider perspective familiar with fused multiply-add ("MAD") operations, I didn't realize that "scale" meant "multiply" and "bias" meant "add" until I took a closer look.

[ngxson]

multiply-add can be confused because we already had ggml_mul and ggml_add which takes 2 tensors as input

ggml_scale on the other hand, takes a tensor and a scalar value as input

So ggml_scale_bias is the best fit because the naming doesn't clash with "multiply" or "add". Bias meaning we add a scalar value, not a tensor