unify `stages.Executable` and `stages.XlaExecutable`
We no longer have many different implicit types conforming to `Executable`, only `pxla.MeshExectuable` and `pxla.PmapExecutable`. Both are `XlaExecutable` subtypes. So define just one common base class, call it `Exectuable`, and inherit from just that in both concrete internal executable subtypes.
PiperOrigin-RevId: 746706712