[perf] Auto causal <> sparse

[blefaudeux]

🚀 Feature

When applicable, automatically use sparse or blocksparse for causal attention. Right now this requires that people use them explicitly, even if the causal flag is passed, which means that a lot of people could miss the possible optimization.

Motivation

Free perf on the table, can be a little complex to handle all cases, but would make sense to do it directly in xFormers.

Pitch

Sort out the applicable cases first, and in that case defer to the sparse or blocksparse when ScaledDotProduct is used with the causal flag

Alternatives

Warn users of this possible optim

Additional context