bugfix in distribute_ranks_to_blocks

[sbailey]

This PR fixes a bug in desispec.scripts.zproc.distribute_ranks_to_blocks where it was creating more blocks (i.e. MPI subcommunicators) than requested. e.g.

from desispec.scripts.zproc import distribute_ranks_to_blocks
nblocks, block_size, block_rank, block_num = distribute_ranks_to_blocks(nblocks=3, rank=63, size=64)
print(f"{nblocks=} {block_size=} {block_rank=} {block_num=}")

main   --> nblocks=3 block_size=21 block_rank=0 block_num=3
thisPR --> nblocks=3 block_size=21 block_rank=20 block_num=2

note that the main returned block_num=3 is too big; it should have been 0, 1, or 2 based upon the requested number of nblocks=3. The consequence was that the GPU node (size=64) zproc script was creating 4 blocks to process 3 afterburners, and there is a race condition with ranks from two different blocks both running the MgII afterburner.

FWIW, I made a similar bug in distributing MPI ranks to GPU devices in PR #1899, so I was familiar with the fix already :)

The key difference is that block_num = int(rank / (size/nblocks)) instead of block_num = int(rank / int(size/nblocks)) with the side effect that different blocks can have different numbers of ranks if size/nblocks isn't an integer, i.e. you can't pre-decide a single integer block_size that applies to every block and still have that use up all the ranks if size/nblocks is not an integer.

This PR includes both the bug fix, and unit tests that fail on current main but pass with this PR. I also tested with running a coadd-redshift job to verify that it also works in practice.

I intend to self-merge since I need this fix for other healpix redshifting development work, but I'm submitting this as a separate PR since it isn't specific to the healpix changes.