[dtensor] PART 1: move DeviceMesh and placement to core distributed (#88549)
This PR creates `torch.distributed._tensor` package and moves
DeviceMesh, PlacementTypes to it
part of https://github.com/pytorch/pytorch/issues/88838
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88549
Approved by: https://github.com/fduwjj
