Fix backwards compatibility for optional logging dependencies

[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?
• If you made a notable change (that affects users), did you update the CHANGELOG?

What does this PR do?

Fixes #798, #799 and #815

There's other ways to fix this, but I felt that adding these redirect files was the simplest and easiest to understand.

[pep8speaks]

Hello @tullie! Thanks for updating this PR.

• In the file pytorch_lightning/logging/comet.py:

Line 2:101: E501 line too long (103 > 100 characters)

• In the file pytorch_lightning/logging/mlflow.py:

Line 2:101: E501 line too long (103 > 100 characters)

• In the file pytorch_lightning/logging/neptune.py:

Line 2:101: E501 line too long (103 > 100 characters)

• In the file pytorch_lightning/logging/test_tube.py:

Line 2:101: E501 line too long (103 > 100 characters)

• In the file pytorch_lightning/logging/wandb.py:

Line 2:101: E501 line too long (103 > 100 characters)

Comment last updated at 2020-02-20 00:40:03 UTC

[Borda]

LGTM 🚀

[williamFalcon]

@tullie thanks!