Added gradient clip test for native AMP

[teddykoker]

What does this PR do?

Avoid repeat of #1381

Before submitting

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• Did you make sure to update the documentation with your changes?
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PR review

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[pep8speaks]

Hello @teddykoker! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-09-30 21:17:33 UTC

[codecov]

Codecov Report

Merging #3754 into master will increase coverage by 7%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #3754    +/-   ##
=======================================
+ Coverage      83%     90%    +7%     
=======================================
  Files         108     108            
  Lines        8807    8491   -316     
=======================================
+ Hits         7284    7608   +324     
+ Misses       1523     883   -640