make plugin type check more flexible

[jedyang97]

What does this PR do?

The current type check of plugins for Trainer is restricted to list by isinstance(plugins, list). However, if plugins is instantiated from something like Hydra or OmegaConf, it will have type <class 'omegaconf.listconfig.ListConfig'>, and the current logic would wrap that list-like object with a second-layer list and cause subsequant logic to fail. If you look at the code, it is not necessary that plugins must be a list - it suffices as long as it is iterable. Therefore, the proposed simple fix change the check from isinstance(plugins, list) to isinstance(plugins, Iterable). I hope this can benefit those who uses Hydra or OmegaConf.

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📚 Documentation preview 📚: https://pytorch-lightning--20186.org.readthedocs.build/en/20186/

[jedyang97]

Ah, I just realized that one can pass in _convert_: partial in OmegaConf to avoid casting the type to <class 'omegaconf.listconfig.ListConfig'>. In this case, I will leave it to the authors to decide whether this change is still worth it.

[awaelchli]

@jedyang97 I'm ok with it either way. However, if this PR should land then we would want to update the type in the signature. And apply the same changes to fabric/connector.py as well.

On the other hand, we have arguments in other places where the user can pass in a list or tuple of elements, and nothing would prevent it to be a general Iterable but we still prefer to advertise the simple types just because of simplicity so it's clear to the user what they can pass in.