CUDA: Fix data-races when reusing SMEM in block_reduce (llama/26385)

CUDA: Fix data-races when reusing SMEM in block_reduce (llama/26385)

* CUDA: Fix data-races when reusing block_reduce

block_reduce currently doesn't resync after reading from SMEM, causing
potential data-races when reusing SMEM for multiple reductions.

One may consider simply always adding this in block_reduce, but this
comes at a potential perf cost

* double-buffering for single-row softmax

* double-buffering for norm as well

* Add comment

* Add explanatory comment to block_reduce

* Specify need for + do memory barrier only in multi-warp scenario

* Implement review-suggestion from @gaugarg-nv