fix trainer not resetting lightning_optimizers

[awaelchli]

What does this PR do?

When calling Trainer.fit multiple times there is a bug where in the second fit the optimization would use the
lightning optimizers from the first model because it is still hanging on to the old list.

The fix is to reset the list when new optimizers are requested from the model.
This way the lightning optimizers get re-wrapped when the training loop starts.

Fixes #6285

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[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 2021-03-06 17:27:02 UTC

[codecov]

Codecov Report

Merging #6372 (a16a75b) into master (4f391bc) will decrease coverage by 2%.
The diff coverage is 92%.

@@           Coverage Diff           @@
##           master   #6372    +/-   ##
=======================================
- Coverage      93%     92%    -2%     
=======================================
  Files         161     161            
  Lines       11464   11476    +12     
=======================================
- Hits        10706   10513   -193     
- Misses        758     963   +205