Remove AccleratorConnector.num_ipus and deprecate Trainer.ipus

[DuYicong515]

What does this PR do?

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

AcceleratorConnector.num_ipus was not meant to be public. It's removed from AcceleratorConnector and deprecated Trainer.ipus in favor of using Trainer.num_devices if isinstance(Trainer.accelerator, IPUAccelerator) else 0

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

None

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Did you have fun?

Make sure you had fun coding 🙃

[krshrimali]

LGTM, thanks @DuYicong515! Left minor comments, can you please take a look?

[tchaton]

LGTM !