What does this PR do?

• Cleans up some of the OptimizerConnector logic to avoid duplicated code.
• Checks if there are extra keys in the lr_scheduler dict. Adds a test for this warning.
• Cleans up the init_optimizers function so configure_optimizers is only called at the end.
• Fixes bug where if configure_optimizers returned ([optimizer], scheduler), trainer.optimizers became [optimizer, scheduler]
• Updates exception message for invalid configure_optimizers returns. Adds a test for this exception.
• Updates the default scheduler config to include strict.
• Check for extra unsupported keys in the lr_scheduler dict. Adds a test for this warning.
• Improves logic to check if there is a monitor when a ReduceLROnPlateau scheduler is used. both in the case of {"optimizer": optimizer, "lr_scheduler": scheduler, "monitor": "your_loss"} and {"optimizer": optimizer, "lr_scheduler": {"scheduler": scheduler, "monitor": "your_loss"}}. Adds tests for these exceptions.
• Updates docs to reflect how we no longer support automatically monitoring val_loss.

No issue to tag. Just some stuff I found while coding #3586

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