[ragged-paged-attn] Unify kv strided load to one.
I expected Mosaic can canonicalize 2 same strided loads to one but it did not. (We will fix this in Mosaic). For now, manually converting to one strided load boosts 20~35% speedup in both v6e and v5e single chip for Meta-Llama-3-8B.
PiperOrigin-RevId: 745294058
