Remove Strategy.on_gpu

[ananthsub]

What does this PR do?

Part of #10416

Reasons for removal:

• This can be deduced from the root_device property. The property on_gpu is redundant yet it's also an abstractmethod. Requiring every strategy class to implement this is boilerplate.
• We don't offer on_ipu or on_X for existing accelerator implementations
• Given the Accelerator is also customizable/pluggable, the pattern of offering on_gpu and on_tpu flags doesn't scale.

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

Yes: removes the on_gpu property from the Strategy base class.
Reason: The strategy is still an experimental API, and this is part of making it stable

See #11536 for the equivalent PR removing on_tpu

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[awaelchli]

why did you choose to check it this way? in some places it may be necessary to differentiate between the model already being on gpu or not yet, but here and in most cases it is only about which accelerator is selected. So an isinstance check would have been more appropriate I believe.