The MagiAttention LocalAttention layer was hardcoded to TORCH_SDPA. The
attention dispatch already routes through FastVideo's selector, so
adding FLASH_ATTN to the supported list lets bf16 inference paths pick
up Hopper FA-3/FA-4 (or FA-2 elsewhere) automatically. Falls back to
SDPA for fp32 (parity tests) which the selector handles cleanly.
Switch the SSIM test to FLASH_ATTN since that's the production
inference backend; pin the parametrize list to a single backend so the
seeded reference videos correspond to the path users actually run.
All 8 runnable magi-human parity tests still pass under
FASTVIDEO_ATTENTION_BACKEND=FLASH_ATTN.
will
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