Rename
* Op mesh.cluster -> mesh.mesh
* Op mesh.cluster_shape -> mesh.mesh_shape
* variables and attributes.
The name `mesh` is more specific to what it really represents. It is a
mesh of devices.
The name `cluster` implies a broader posibility of device
configurations. When just the word `mesh` is used the meaning can often
be inferred from the context whether it refers to the mesh dialect or a
device mesh. The full name can be used when needed.
