Set _choose_auto_accelerator to staticmethod

[fedebotu]

What does this PR do?

This is a minor change. It sets the _choose_auto_accelerator as a staticmethod since no attribute of self is used.

This can also be useful for other libraries to check whether hardware accelerators are available via _AcceleratorConnector._choose_auto_accelerator() without the need to reimplement this function!

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📚 Documentation preview 📚: https://pytorch-lightning--19822.org.readthedocs.build/en/19822/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59%. Comparing base (5e0e02b) to head (f90a79a).
Report is 40 commits behind head on master.

Additional details and impacted files

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##           master   #19822     +/-   ##
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- Coverage      84%      59%    -25%     
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  Files         424      421      -3     
  Lines       34698    35160    +462     
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- Hits        29153    20758   -8395     
- Misses       5545    14402   +8857     

[awaelchli]

@fedebotu As a clean up, LGTM, thanks!

This can also be useful for other libraries to check whether hardware accelerators are available via _AcceleratorConnector._choose_auto_accelerator() without the need to reimplement this function!

That's great if it's helpful to you. However I would not rely on this internal method. Protected marked methods are internal to Lightning and can change anytime without warning, potentially breaking your code.