ggml-webgpu: improve flastAttention performance by software pipelining (#19151)

ggml-webgpu: improve flastAttention performance by software pipelining (#19151)

* webgpu : pipeline flash_attn Q/K loads in WGSL

* ggml-webgpu: unroll Q*K accumlation inner loop

* ggml-webgpu: vectorization

* ggml-webgpu: unrolling

* ggml-webgpu: remove redundant unrolling

* ggml-webgpu: restore the config

* ggml-webgpu: remove redundant comments

* ggml-webgpu: formatting

* ggml-webgpu: formatting and remove vectorization

* ggml-webgpu: remove unnecessary constants

* ggml-webgpu: change QKV buffer to read_write to pass validation

* ggml-webgpu: add explanation for the additional bracket around Q K accumulate

* Indentation and for -> if for tail

* Kick off CI on wgsl only commits

---------

Co-authored-by: Reese Levine <redacted>