[DCP] Enable nD device_mesh resharding DTensor in DCP and add associated tests (#106230)
This PR:
1. Drop assert for 1D DeviceMesh check to allow DTensor with nD DeviceMesh when creating write_item.
2. Add tests for both placement changes and mesh changes for both 1D and 2D scenarios.
cc. @kumpera @wanchaol @fegin
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106230
Approved by: https://github.com/kumpera
