hexagon: further optimizations and refactoring for flash attention (#19583)

hexagon: further optimizations and refactoring for flash attention (#19583)

* ggml-hexagon: fa improvements

ggml-hexagon: optimize flash attention calculations with improved variable handling

ggml-hexagon: streamline flash attention operations by removing redundant checks for FP32

ggml-hexagon: optimize hvx_dot_f16_f16_aa_rx2 by simplifying variable handling for unused elements

ggml-hexagon: optimize flash attention by changing slope vector type to F16

* hexfa: fixed test-backend-ops failurs due to leftover element handling

* hexagon: refactor and optimize fa to use local context struct

* ggml-hexagon: optimize flash-attention using hvx_vec_expf

Use HVX for online softmax.

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Co-authored-by: chraac <redacted>