CUDA : Fix CUB's argsort when nrows % block_size == 0 CCCL < 3.1 (llama/21181)

CUDA : Fix CUB's argsort when nrows % block_size == 0 CCCL < 3.1 (llama/21181)

* CUDA: Fix CUB's argsort when nrows % block_size == 0 CCCL < 3.1

We wrongly calculated offset_grid as `ceildiv(nrows, block_size)`,
while it must be `ceildiv(nrows + 1, block_size)`. As a consequence, we
had uninitialized values in `offset_iterator[nrows]` for the case when
`nrows % block_size == 0`.

Fixes #21162

* Reduce nrows in test case to 256, don't need 768