SDPA: frontend for BSR masks (#104042)
This PR implements a (yet private) frontend for scaled_dot_product_attention that works with BSR `attn_mask`.
This function is directly comparable (with suitable masks) with `torch.nn.functional.scaled_dot_product_attention` once `attn_mask.dtype == torch.bool`, but it's behavior is different when `attn_mask.dtype != torch.bool`. This is because `torch.nn.functional.scaled_dot_product_attention` assumes that irrelevant values are supposed to be filled with `-inf`, while the selected ones should be `0`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/104042
Approved by: https://github.com/amjames, https://github.com/cpuhrsch
