Fix optimizer_closure return result

[alexeykarnachev]

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What does this PR do?

Fixes #1510 .

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[codecov]

Codecov Report

Merging #1516 into master will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1516   +/-   ##
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- Coverage      91%     91%   -0%     
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  Files          67      67           
  Lines        3784    3784           
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- Hits         3439    3438    -1     
- Misses        345     346    +1     

[williamFalcon]

@alexeykarnachev
This fix won't work because the output of this goes into
training_epoch_end, which means things have to be tensors.

I'll submit a different PR to fix

[williamFalcon]

Continuted in: #1528