vulkan: Multi-pass softmax for large number of cols (llama/17892)

vulkan: Multi-pass softmax for large number of cols (llama/17892)

When the number of cols is large, split each row across multiple workgroups.
There are three phases that communicate partial results through temp buffers:
(1) compute max partials
(2) take max of partials, compute sum(exp(x-max)) partials
(3) sum partials, compute scaled result