[ExecuTorch][WebGPU] Add fused SDPA (sdpa_with_kv_cache) with dynamic input_pos (#20259)

This PR was created by the merge bot to help merge the original PR into
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ghstack PR number: #20086 by
@JulianCloudNTH
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ghstack PR base:
https://github.com/pytorch/executorch/tree/gh/JulianCloudNTH/19/base
ghstack PR head:
https://github.com/pytorch/executorch/tree/gh/JulianCloudNTH/19/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head:
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Co-authored-by: Julian Ng-Thow-Hing <juliannth@meta.com>

Author: pytorchbot