Remove AcceleratorConnector.tpu_cores and Deprecate Trainer.tpu_cores

[DuYicong515]

What does this PR do?

Part of #11449 : remove unused properties in accelerator_connector: tpu_cores
Part of #12171 : deprecate device related properties in Trainer that is accessed from accelerator_connector: tpu_cores

AcceleratorConnector.tpu_cores was not meant to be public. It's removed from AcceleratorConnector and deprecated in Trainer in favor of using Trainer.num_devices

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

None

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