[DTensor] Fix DeviceMesh (#96861)
Summary: This Diff fixes some DeviceMesh issues, which blocks internal DTensor integration. Specifically, when `self.mesh = [2, 3]` while `world_size = 4`, because `unique_mesh_values[-1] == 3`, it takes the first short-cut branch and uses `default_pg`. Let's check the length instead of the last value of `unique_mesh_values`.
Test Plan: CI
Reviewed By: wanchaol
Differential Revision: D44079872
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96861
Approved by: https://github.com/wanchaol
