[DTensor] Fix the default PG condition for DeviceMesh (#97384)
The current conditin to use the default PG is `len(unique_mesh_values) == WORLD_SIZE - 1`. The `- 1` is not correct and seems to be an incorrect fix from https://github.com/pytorch/pytorch/pull/96861.
Differential Revision: [D44314317](https://our.internmc.facebook.com/intern/diff/D44314317/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97384
Approved by: https://github.com/wanchaol
