ggml-webgpu: enable FLASH_ATTN_EXT on browser without subgroup matrix (llama/22199)

ggml-webgpu: enable FLASH_ATTN_EXT on browser without subgroup matrix (llama/22199)

* ggml-webgpu: add tile flash attention fallback

* ggml-webgpu: add new fields and discard usage of mnk for tile version

* ggml-webgpu: modify the vec path to discard the mnk parameter

* ggml-webgpu: enable flash attention vec and tile version for broswer

* ggml-webgpu: stagging KV for flash attention tile version

* formatting

* turn on subgroup uniformity check

* remove Q_TILE as it is always 1 for vec path

* make row_max and exp_sum to local register

* make different bindings with same underlying buffer to have the same usage flags

* move path selection into the shader library and have the host consume a single flash-attn decision object.

* turn off skip_validation and address buffer overlapping when nwg==1

* formatting

* merge binding when kv overlap