Optional dependencies are required for deprecated logging module

[tullie]

🐛 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

1. Remove all logging dependencies from your environment (E.g. comet)
2. 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.

[Borda]

Not sure if I understand correctly so if you remove eg mlflow and import pytorch_lightning.logging it crashes with an import error?
@tullie how is it related to issue 155 linked in bug title?

[tullie]

Sorry it isn't related to 155. Not sure how that slipped in there.

Otherwise, your understanding is right.

[Borda]

Sorry it isn't related to 155. Not sure how that slipped in there.

The tricky thing is when it creates issue/PR it enters the text field starting with # which is associated with an issue/PR number so it automatically opens suggestion pop-up and the oldest issue (on the top) is the 155 and without enough attention, it is placed there automatically... It happened me several times already :]
@williamFalcon could you grant to @PyTorchLightning/core-contributors right to edit issues/PR so we do not border users with simple-straight-forward issues... 8-]