Fix weights_save_path when logger is used + simplify path handling + better docs

[awaelchli]

What does this PR do?

Fixes #2527, #882, #1812 and continues effort to document and protect Trainer attributes #1497 better.

1. If user defines the Logger save dir, it will now properly work with the setting for weights_save_path.
2. Makes sure that the weights_save_path only gets picked if it actually was changed by user.
3. Fixed weights_save_path not getting used when logger=False
4. Deprecated trainer.ckpt_path since it was never actually different from weights_save_path, and never used anywhere in the code base.
5. Made sure that the checkpoint callback does not set the properties like weights_save_path, it's not needed and prevented multiple checkpoint callbacks being used.
6. Added detailed docs about path settings

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together? Otherwise, we ask you to create a separate PR for every change.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

[pep8speaks]

Hello @awaelchli! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-07-26 22:50:11 UTC

[williamFalcon]

without this what will be the path? just none?

[awaelchli]

no, it will still be default_root_dir, it's just getting set in the Trainer init now.

[williamFalcon]

@awaelchli make sure to list the Fixed in different lines so the GH issues close automatically

Fixes a
Fixes b
...

[awaelchli]

ok, got it, did not know, thanks.