[PTD][Checkpoint] Add checkpointing support for DTensor submesh (#96802)
DTensor submesh support is added in https://github.com/pytorch/pytorch/pull/95458.
This PR adds support for DTensor submesh by adding an extra check when create local save/load plan.
If the rank is not participating in the mesh, we simply skip creating WriteItem/ReadItem for the local SavePlan/LoadPlan.
Updated the associated test as well.
cc. @wanchaol, @kumpera
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96802
Approved by: https://github.com/wanchaol
