[Minor] 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/