Add Accelerator.is_available() interface requirement

[ananthsub]

What does this PR do?

• Enables automatic hardware selection without duplicating code across Trainer & individual accelerator implementations. This can be further enhanced by the addition of an AcceleratorRegistry. The accelerator connector could iterate through all of the registered accelerators, call acc_cls.is_available() and determine the hardware immediately.

This aims to simplify the accelerator connector logic and rewrite effort in #11448
This would move the hardcoded, duplicated assertion logic from Trainer constructor checks to a single runtime check at trainer.fit/validate/test/predict calls

Given discussion on this PR, we can decide where the assertion on device availability should happen. Either in the accelerator init, setup_environment, or left up to individual accelerators to decide

Fixes #11818

Does your PR introduce any breaking changes? If yes, please list them.

Yes, this now:

• raises a TypeError if a custom accelerator has not implemented the new abstract method
• raises a RuntimeError if the configured hardware is not available during Trainer execution

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[carmocca]

Does this close #11799 and #11798?

just need some clarifications

[ananthsub]

Does this close #11799 and #11798?

Yes, I will close them after this is merged