qwen4exp: sum the indexer heads by slices (#28023)
* qwen4exp: sum the indexer heads by slices
The head reduction went through a transpose and a sum_rows over ne[1],
which left sum_rows with ne0 = 4, one block per row for a four element
reduction, and the transpose copied the whole block by token surface
twice on the way in.
The heads are adjacent on ne[1], so each one is a strided view and the
sum is a short chain of adds.
RTX PRO 6000, Qwen3.8-Flash-Next UD-Q4_K_XL, fa on, 55k context, warm
runs on top of #28011:
prompt processing 2170 -> 2366 t/s
Generation is unaffected. The removed work scales with n_blocks by
n_tokens, so the gain grows with context and with ubatch size.
* qwen4exp: drop the redundant cont on the indexer query
rope returns a freshly allocated, contiguous tensor, so the reshape that
feeds the matmul does not need a copy. ggml_reshape_3d asserts
contiguity, so a layout that would need the cont cannot slip through
silently.
Greedy output is unchanged token for token.
Address review from @ggerganov
