Arm backend: Add fused i8 warp+downsample example

[oscarandersson8218]

Adds example where int8 warp+downsample is fused into one custom shader. This pattern is present in rife model and such a fusion can reduce memory bandwidth significantly as it avoids processing pixels that are not used by downsampling.

cc @digantdesai @freddan80 @per @zingo @mansnils @Sebastian-Larsson @robell @rascani

[pytorch-bot]

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[zingo]

nit: Maybe also update examples/arm/QAT_example/README.md

[zingo]

I assume the bin files are used/needed and in this stage not generated?

[oscarandersson8218]

I assume the bin files are used/needed and in this stage not generated?

They are needed just like the ones in https://github.com/pytorch/executorch/tree/main/backends/arm/vgf/shaders.

nit: Maybe also update examples/arm/QAT_example/README.md

I'll add this in a follow up if that's ok.