Fix `distributed_concat` with scalar tensor
Author: Yard1
Pull request: yes

# What does this PR do?

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If a scalar tensor is passed to `distributed_concat`, the output tensors are correctly converted to one element vectors. However, this is not done for the tensor itself, which causes an exception to be thrown in `dist.all_gather` due to a tensor length mismatch. This PR fixes that.

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Comments:
cc @sgugger 

_The documentation is not available anymore as the PR was closed or merged._