[SP] add SP deny list instead of allow

[kashif]

this way one can register kernels based flash-attn as well with SP

[tohtana]

Hi @kashif,

Thank you for opening this PR! I think supporting HF hub kernels is is a significant update.

Regarding the approach, we check if core_attn_implementation is in ALL_ATTENTION_FUNCTIONS but HF hub kernels like kernels-community/flash-attn2 is not in the list. So HF hub kernels won’t still be available with this fix.

We probably need to do the proper registration steps:

1. Reject known-bad impls explicitly: eager, flex_attention, and probably paged|eager.
2. If core_attn_implementation is an HF hub kernel string, call the HF registration path first. (Using lazy_import_flash_attention(…))
3. Then read core_attn_function = ALL_ATTENTION_FUNCTIONS[core_attn_implementation].
4. Build uattn from that original function.
5. Replace that key with uattn_wrapper.

Does it make sense to you?