horovod bug: conflict of group argument in all_gather function

[marsggbo]

🐛 Bug

...
from pytorch_lightning.utilities.distributed import group, rank_zero_only, ReduceOp
...

class HorovodPlugin(ParallelPlugin):
    ....

    def all_gather(
        self,
        result: Union[torch.Tensor],
        group: Optional[Any] = group.WORLD,
        sync_grads: bool = False
    ) -> torch.Tensor:
        if group is not None and group != group.WORLD:
            raise ValueError(
                "Horovod does not support allgather using a subcommunicator at this time. "
                "Unset `group`."
            )

        if len(result.shape) == 0:
            # Convert scalars to single dimension tensors
            result = result.reshape(1)

        # sync and gather all
        self.join()
        gathered = hvd.allgather(result)
        gathered_result = list(gathered.split(1, dim=0))
        return gathered_result

we can see that the input argument group is conflicted with the imported module group

[tchaton]

Dear @marsggbo,

Thanks for reporting this issue. Would you mind to make a PR ?

Best,
T.C

[marsggbo]

PR #7840 solves this problem. I just simply replace module group with GROUP

[awaelchli]

@marsggbo thanks for your help!