Bugfix/lr finder

[SkafteNicki]

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• Did you write any new necessary tests?
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What does this PR do?

Fixes #1648 where the learning rate finder could not be used by users that prefer to pass val_dataloaders directly to the different methods.

Fixed #1650 where the learning rate finder could not be used, when a EarlyStopping callback is passed to trainer.

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.

Did you have fun?

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[awaelchli]

Looks ok :)
What I forgot to ask in the original PR for the LR finder: Isn't the dumping and restoring of Trainer args a bit fragile? If we introduce new Trainer args that interfere with the LRFinder, we always have to remember adding them here for dumping and restoring? Otherwise it will introduce a bug. Or do I misunderstand the purpose?

[SkafteNicki]

The idea behind dumping and restoring trainer args is to take advantage of fit during the learning rate search (alternative would be to write a new training loop). I agree that this is a fragile feature, however for me the clear advantages of calling fit internally outweigh these disadvantages

[mergify]

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

[awaelchli]

@SkafteNicki it makes sense to use fit, that's very good :) we just have to keep it in mind when we add new trainer args :) 👍

[codecov]

Codecov Report

Merging #1676 into master will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1676   +/-   ##
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  Coverage      88%     88%           
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  Files          69      69           
  Lines        4131    4135    +4     
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+ Hits         3655    3659    +4     
  Misses        476     476           

[mergify]

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