Fix how NCCL existence is checked

[tharittk]

The bug is discovered by @mrava87 when the system is installed with CuPy but without NCCL.

Scenario:
CuPy always comes with cupy.cuda.nccl so checking the import error of cupy.cuda.nccl is not sufficient to verify the existence of NCCL. Under the original code, with CuPy installed without NCCL, the deps.nccl_enabled will be True (as no import error occurs). This will lead to crash import pylops_mpi because it can't find nccl.NCCL_FLOAT32

Fix:
If CuPy is present, this import statement import cupy.cuda.nccl as nccl will not throw an error and we can check the NCCL's existence through this nccl.available flag

[mrava87]

@tharittk this looks great, I think this should solve the problem that we saw when cupy is installed but nccl is not . I am going to merge this 😄