[XLA:Python] Rework local backend registration.
[XLA:Python] Rework local backend registration.

Add a get_local_backend() that selects a good local backend automatically.
Add register_local_backend() which allows clients to plug in extra backends.
Use more natural "cpu" and "gpu" names to name backends.

Remove some deprecated logic to instantiate backends that existed for backward compatibility with older Jax clients. Remove the old replica count logic.

Remove XRT backend temporarily in preparation for a future change.

PiperOrigin-RevId: 241059188

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