Create loggers property for Trainer and LightningModule

[akashkw]

What does this PR do?

Implement a loggers property that returns a list of loggers. This is meant to replace LoggerCollection, which will then be deprecated.

Part of #11232

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

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[daniellepintz]

Looks good!

[akashkw]

We are going to handle the special case for the loggers setter where someone assigns trainer.loggers = [LoggerCollection]
This will make it easier to refactor the codebase such that anywhere the user passes in a logger, we can easily put it in a list and assign it to trainer.loggers.

If we try to manually check and sanitize this everywhere it occurs in the codebase, it will be harder to deprecate LoggerCollection and trainer.logger. Sanitizing the input to trainer.loggers would also remove information about whether the user provided a LoggerCollection, which would lead to incorrect behavior if the LoggerCollection is of size 1. There are multiple solutions to this problem but, in my opinion, they are all messier than adding a special case to the setter.

[daniellepintz]

Could someone please rerun the CI and enable auto-merge?

[daniellepintz]

Can we update this PR to include the proposal in #11232 (comment)?

[daniellepintz]

Looks great! Thanks for your persistence on this. I think it's ready to merge