test: started adding tests for nccl utility routines

[mrava87]

This is a WIP PR to add tests for various nccl utility routines in pylops_mpi.utils._nccl for primitive communication calls.

The need for this tests stems from the realization that if we pass send_buf of different size to nccl_allgather we get a deadlock... so we should make sure to check and raise an error before this happens.

Note that the last test is currently commented out because mpiexec will hang if running this test - I believe this is due to the fact that nccl_comm.allGather does not cope with send_buf of different size between ranks

[mrava87]

Superseded by #148