metal : PP speedup (#3084)

metal : PP speedup (#3084)

* Minor speed gains for all quantization types

* metal: faster kernel_scale via float4

* Various other speedups for "small" kernels

* metal: faster soft_max vial float4

* metal: faster diagonal infinity

Although, to me it looks like one should simply
fuse scale + diagnonal infinity + soft_max on the
KQtensor.

* Another faster f16 x f32 matrix multiply kernel

* Reverting the diag infinity change

It does work for PP, but somehow it fails for TG.
Need to look more into it.

* metal: add back faster diagonal infinity

This time more carefully

* metal : minor (readibility)

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Co-authored-by: Iwan Kawrakow <redacted>
Co-authored-by: Georgi Gerganov <redacted>