properly save model across tensor parallel processes
Author: winglian
Pull request: yes

# What does this PR do?

Tensor Parallel saving doesn't work because currently it would use the default of checking args.should_save which would only save on rank0, but we need to coordinate across TP ranks to gather the dtensors.

This is still in draft as I need to solve for FSDP + TP too.

@S1ro1 
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Fixes # (issue)


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Comments:
Superseeded by #39693