[XLA:Python] Refactor XLA client to support multiple platforms in the same process.

Use device ordinals when naming devices for execution, buffer placement, infeed, and outfeed. A replica is a property of a computation, and computations may have different numbers of replicas. Different numbers of replicas may be mapped onto a particular topology of devices differently, so a replica number does not uniquely name a device.

PiperOrigin-RevId: 233962949