Add support to Tensorboard logger for OmegaConf hparams

[ananthsub]

What does this PR do?

Fixes #2844
We check if we can import omegaconf, and if the hparams are omegaconf instances. if so, we use OmegaConf.merge to preserve the typing, such that saving hparams to yaml actually triggers the OmegaConf branch

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[romesco]

Check #2519, I think this PR which has been in limbo already fixes #2844 🤙

[mergify]

This pull request is now in conflict... :(

[codecov]

Codecov Report

Merging #2846 into master will increase coverage by 0%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #2846    +/-   ##
=======================================
  Coverage      90%     90%            
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  Files          79      79            
  Lines        7233    7488   +255     
=======================================
+ Hits         6527    6767   +240     
- Misses        706     721    +15     

[awaelchli]

has anyone checked if the other loggers support omegaconf out of the box?

[awaelchli]

just wanted to point out (nitpick) that importerror is not the right exception to catch. What you try to do here is check if the package is available, which can be done without trying to import it with just one line with importlib. If the package is available, there can still be an importerror for a different reason. In such a case we really WANT to see the stacktrace and not just ignore it. See also #2266

This actually occurs in many places of PL. Would be great to clean up these code smells and eliminate them right away in the review process of new PRs @PyTorchLightning/core-contributors