fix LightningTemplateModel

[festeh]

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

Fixes #1546. Now it's possible to run Trainer.test() for LightningTemplateModel

PR review

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[williamFalcon]

this example is very old.
We really should just remove it or replace it with
https://colab.research.google.com/drive/1-_LKx4HwAxl5M6xPJmqAAu444LTDQoa3#scrollTo=8kaxQli2ahpR

[williamFalcon]

let's make the correct changes instead :)

[codecov]

Codecov Report

Merging #1577 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1577   +/-   ##
======================================
  Coverage      88%     88%           
======================================
  Files          71      71           
  Lines        4175    4175           
======================================
  Hits         3692    3692           
  Misses        483     483           

[mergify]

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

[festeh]

I find it useful to have a simple example in the repository, sometimes it helps me to do some quick experiments. So I updated this module to look more like Colab version.