vulkan: Optimize GGML_OP_CUMSUM (llama/18417)

vulkan: Optimize GGML_OP_CUMSUM (llama/18417)

* vulkan: Optimize GGML_OP_CUMSUM

There are two paths: The preexisting one that does a whole row per workgroup
in a single shader, and one that splits each row into multiple blocks and does
two passes. The first pass computes partials within a block, the second adds
the block partials to compute the final result. The multipass shader is used
when there are a small number of large rows.

In the whole-row shader, handle multiple elements per invocation.

* use 2 ELEM_PER_THREAD for AMD/Intel

* address feedback