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metal: add the 128/96 flash attention kernels - #30209
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Overview
#30003 added flash attention tests with K = 128 and V = 96, and Metal crashed on them: it said it supported the pair, but had no kernel for it.
Instead of rejecting the pair (#30208), this adds the 128/96 kernels, the same way 96/64 is done (mma and vec, every K/V type Metal handles). supports_op now accepts exactly the pairs that have a kernel: K == V, plus 96/64, 128/96, 192/128, 320/256 and 576/512. The vec tuning baseline and the sparse path list get 128/96 too.
Tested on an M5 Max: test-backend-ops -o FLASH_ATTN_EXT passes 4973/4973 on Metal, the 128/96 cases run on Metal for every K/V type it supports, and 96/64, 192/128, 320/256 and 576/512 keep running on Metal. No model uses 128/96 yet, so the test suite and the CI are the coverage here.
Additional information
Supersedes #30208
Follow-up #30003
Requirements