Add auto_device_count method to Accelerators

[kaushikb11]

What does this PR do?

Follow up PR will be adding support for devices="auto", where the accelerator will have the info what’s the maximum devices available to train on.

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

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[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[carmocca]

(Copying this because we commented at the same time)

@carmocca Then how could we have it as abstract then?

you could "simulate" it by having it return it Optional[int] = None in "Accelerator" and checking that it's not None when you use the value

No strong feelings though. Up to you

[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>