[dtensor] refactor get_coordiniate (#95457)
This refactor get_coordinate to return a optional[list] instead of
directly the coordinate on dim, this is so that we can check if the
rank is inside the mesh easily
Differential Revision: [D43643579](https://our.internmc.facebook.com/intern/diff/D43643579)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95457
Approved by: https://github.com/XilunWu
