Reorder channelwise gated delta rule chunked hot loops for autovectorization (#21021) (#21021)

Summary:

Reorder the chunked prefill inner loops (steps 1, 4, 5, 6) so the
innermost loop runs contiguously over the head dimension (k or v)
instead of striding down a column of the state / pv. This lets the
compiler autovectorize the now-unit-stride AXPYs; hand-written at::vec
was tried and was slower than the compiler output, so the loops stay
scalar.

Reviewed By: billmguo

Differential Revision: D112598714