[BUG] Fix num_cta_per_query div

[lowener]

This bug happened when trying to run a CAGRA index with itopk=100 and topk=100. The num_cta_per_query variable was equal to 3 because 100 / 32 = 3.125 instead of ceildiv(100, 32) = 4. This resulted in the following error:

RuntimeError: RAFT failure at file=/opt/conda/conda-bld/work/cpp/include/raft/neighbors/detail/cagra/search_multi_cta.cuh line=183:
`num_cta_per_query` (3) * 32 must be equal to or greater than `topk` (100) when 'search_mode' is "multi-cta". (`num_cta_per_query`=max(`search_width`, `itopk_size`/32))

[enp1s0]

Thank you, @lowener, for fixing the bug! The PR looks good to me.

[cjnolet]

/merge