[RFC] Autotune should consider batch size and number of heads

[sustcsonglin]

Proposal

The optimal kernel configuration should adjust based on changes in (batch size × number of heads).

Rationale

The performance of the autotuned kernel can vary significantly when the product of (batch size × number of heads) changes, especially with different levels of parallelism determined by the batch and head dimensions.

[sustcsonglin]

Autotuning should also take the total sequence length into account, as the sequence length dimension provides parallelism in addition to the number of heads and batch size.