Fix backwards compatibility for deprecated logging module

[tullie]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
• Did you read the contributor guideline?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Fixes #798

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

[Borda]

Maybe we do not have open an issue if have already fix... And rather write the description here instead... So there is no no need to swapping between issue and PR =)

[Borda]

it seems that it does fix one hole but breaks other compatibility...

import pytorch_lightning.loggers.mlflow
import pytorch_lightning.logging.mlflow
Traceback (most recent call last):
  File "/home/jb/.local/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 3265, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-10-8c3cb4802809>", line 1, in <module>
    import pytorch_lightning.logging.mlflow
  File "/home/jb//pydev/_pydev_bundle/pydev_import_hook.py", line 21, in do_import
    module = self._system_import(name, *args, **kwargs)
ModuleNotFoundError: No module named 'pytorch_lightning.logging.mlflow'

[tullie]

Yeah agreed that making the issues doesn't make sense. I only did it because the PR description prompted me too.

Good point. I'll revise today to fix that too.

[Borda]

@williamFalcon it was not fixed... @tullie wanted to continue on this...

[williamFalcon]

@Borda oh shoot, sorry didn't read that last comment. Let's finish this in a separate PR?

[Borda]

Well as you already merged it, it has to done in other PR #815?