Optional dependencies are required for deprecated logging module
🐛 Bug
There is a backwards compatibility issues coming from PR #767. Notably, if a user doesn't have any of the extra logging dependencies then they'll be an import error.
To Reproduce

Remove all logging dependencies from your environment (E.g. comet)
Depend on the deprecated pytorch_lightning.logging package and run

Expected behavior
We expect to maintain backwards compatibility here so optional dependencies shouldn't be required.