vulkan: Reduce temporary memory usage for TOP_K (llama/17623)

vulkan: Reduce temporary memory usage for TOP_K (llama/17623)

- Compute row size for the temp buffer based on the output of the first pass.
- Update shader addressing math to use the output row size
- Pass the output row size as "ncols_output", what used to be "ncols_output" is now "k"

For the common case of K=40 and src0=(200000,1,1,1), this reduces the temporary buffer
from about 3.2MB to 500KB.