Let kernel modules declare their preferred mask function
`load_and_register_attn_kernel` hardcodes the mask function to
`flash_attention_2` for all custom attention kernels. This is incorrect
for kernels that need a different mask type (e.g., SDPA-style masks).
Add support for a `MASK_FUNCTION` module-level attribute on kernel
packages. If present, it specifies which mask type to use (e.g.,
"sdpa", "eager"). Falls back to "flash_attention_2" for backward
compatibility when the attribute is absent.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
