Fix semi-sync training with 1GPU per FT replica #1505
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When using semi-sync training with a single GPU per replica, no mesh is created in ParallelDims, causing an error when all-reducing the loss.
The slicing error occurs on this line:
torchtitan/torchtitan/train.py
Line 502 in cf30b29
This PR prevents this from happening by creating a default mesh when no parallelism is used.