Let TorchCollective works on the torch.distributed WORLD process group by default

[belerico]

What does this PR do?

With this PR TorchCollective works by default on the torch.distributed.group.WORLD process group, without the need to first create a new group.

Fixes #16922

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[codecov]

Codecov Report

Merging #16995 (6a9fa30) into master (7bc2a65) will decrease coverage by 23%.
The diff coverage is 85%.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #16995     +/-   ##
=========================================
- Coverage      82%      59%    -23%     
=========================================
  Files         436      413     -23     
  Lines       31765    31486    -279     
=========================================
- Hits        26009    18687   -7322     
- Misses       5756    12799   +7043     

[belerico]

I've seen also that from pytorch 2.0 the torch.distributed.distributed_c10d._pg_map should be called from the _world singleton. I don't know if TorchCollective needs to support something like that, if so one can just add the following

if _TORCH_GREATER_EQUAL_2_0:
    from torch.distributed.distributed_c10d import _world

    pg_map = _world.pg_map
else:
    pg_map = dist.distributed_c10d._pg_map

at the beginning and refers to pg_map where needed.
@carmocca @awaelchli, what do you think? I can update the PR in case it's needed