[2/N][DeviceMesh] Overriding __getitem__ for DeviceMesh to support Mesh Slicing (#107730)
Add support for DeviceMesh slicing by overloading __getitem__ for DeviceMesh.
With this change, you can do:
```
mesh_shape = (2, 4)
mesh_dim_names = ("DP", "TP")
two_d_mesh = init_device_mesh(
self.device_type, mesh_shape, mesh_dim_names=mesh_dim_names
)
tp_mesh = two_d_mesh["TP"]
```
cc. @wanchaol, @fduwjj
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107730
Approved by: https://github.com/wanchaol
