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/