[ExecuTorch][WebGPU] Fuse attention QKV projections in prefill (#21107)

This PR was created by the merge bot to help merge the original PR into
the main branch.
ghstack PR number: https://github.com/pytorch/executorch/pull/20880 by
@JCNTH
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ghstack PR base:
https://github.com/pytorch/executorch/tree/gh/JCNTH/52/base
ghstack PR head:
https://github.com/pytorch/executorch/tree/gh/JCNTH/52/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head:
https://github.com/pytorch/executorch/tree/gh/JCNTH/52/orig

@diff-train-skip-merge

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Co-authored-by: Julian Ng-Thow-Hing <juliannth@meta.com>
Co-authored-by: Julian Ng-Thow-Hing <107437036+JCNTH@users.noreply.github.com>